How Transportation Technologies Shaped Empires

This is Part 5 of the series of how transportation technologies have shaped the world (here are parts 1, 2, 3, and 4). So far, we’ve explored how transportation technologies determined everything about cities. Today, we’re jumping to empires.

The Hidden Rule of Empire Size

For thousands of years, a hidden rule limited the size of empires, unbeknownst to their rulers.

Why did Romans beat hundreds of peoples and expand for centuries, but couldn’t overpower the Germanic Tribes?

Why did Russians conquer everything all the way to the Pacific, and have kept all that land to this day?

Why did Medieval Europe have small feudal states and strong naval states?

What limited the expansion of the Chinese empire? Of Indian empires? The Incas, the Aztecs, the British, the Dutch, the Portuguese, the Russian? 

Napoleon, Genghis Khan, Attila the Hun, Alexander the Great… History is littered with the tales of grand invaders who created empires that crumbled on their deathbed. Why couldn’t these empires last?

Let’s look at three ancient empires to understand what’s going on.

Egypt

As I explained in detail in The Dictatorship of the Nile, before Rome, Egypt had existed for 3,000 years. In that time, Egypt barely expanded.

For the first thousand years, it was just as long as the Nile up to the 1st cataract. The distance between Cairo¹ and that cataract is about 1,000 km. Transport speed on the Nile was anything between 17 and 140 km/day, which means the trip took anywhere from one week to two months.

As you can imagine, passing the first cataract took an additional inordinate amount of time—you had to take the boats out of the water, drag them over rocks, and drop them back into the water. So they didn’t, and the first cataract was for the longest time the natural border of the empire.

You can see how the kingdom was united up to the 1st cataract. Beyond that, it wasn’t. There were a bunch of Nubian kingdoms. Source: Forgotten Wars - The Roman Invasion of Nubia, Invicta.

Even after they managed to figure out how to pass the cataract, they could only make it to the 2nd cataract—same problem again.

Source: Forgotten Wars - The Roman Invasion of Nubia, Invicta

Even then, that stretched Egypt too much. Being months away from the capital was untenable. And that’s why Egypt’s size was always limited by the speed of transport over the Nile.

Since Egypt couldn’t expand south further than the first cataract, and east or west due to the desert, the only way out was north. The Egyptian civilization was based in the delta, so they could travel the sea a bit. But they didn’t have enough wood for lots of ships, so the country was still limited by the speed of their armies on foot. And that’s why, to the north, it could never go beyond the Levant.

The light green is a desert mirage. There was nothing there. 

But the Levant was closer to Mesopotamia (mostly present-day Iraq) and Anatolia (mostly present-day Turkey) so Egypt could not hold that territory for long after empires emerged there. For example, the Achaemenid Empire (Persia) emerged close to Mesopotamia, conquered all the area, and then reached Egypt. The Greek also followed Anatolia, then the Levant, then Egypt, (supported by the sea), like the Romans afterwards (from the sea).

Rome

This is how long it took the average Roman traveler to go from Rome to the rest of the empire—the redder, the more time it took:

Source: Orbis, the Stanford Geospatial Network Model of the Roman World

Compare this to the expansion of the Roman Empire:

Source

Here’s a static map that shows the same thing:

Source

You might notice something: Roman expansion went from what was closest to farthest not in terms of distance, but in terms of transport time. And the natural limit of its expansion corresponded to one month of travel from Rome. Anything beyond that couldn’t be held.

This brings a new light to the two transportation assets Romans were famous for: the Mare Nostrum (Mediterranean) and the roads. The sea allowed for fast travel across the Mediterranean, uniting it—but preventing Rome from going much beyond it. The roads were necessary for Rome to move past the coasts and control the land.

While London, the upper Nile, the Levant, and even the Black Sea could be reached in less than a month, the lands beyond the Rhine river, today’s Germany, couldn’t.

And this is in a world where they had no military or economic equal. As neighboring areas grew stronger, one month of distance was too remote to hold. Rome abandoned Britain, Germanic tribes invaded the European side, the Sasanid Empire on the Asian side, and the half of the empire farthest from Rome split.

Achaemenid Empire

If the Roman Empire owed its final size to the speed of transport by land and sea, why was the Achaemenid Empire (which reigned centuries before Rome, between 550 and 330 BC) one third bigger than the Roman Empire at its peak?

The king kept overall control through a sophisticated system of couriers running and riding on robust horses day and night, winter and summer, over a well-developed network of imperial roads. Within the 2,500 kilometers of highways, the most impressive stretch was the Royal Road from Susa to Sardis, built by command of Darius I. Crucially, relays of mounted couriers could reach the remotest of areas in fifteen days. Royal inspectors, the “eyes and ears of the king,” toured the empire and reported on local conditions.—Quantitative Dynamics of Human Empires, Marchetti, Ausubel.

These are the guys who invaded Egypt from the north.

The Western part of the Achaemenid Empire, along with the Imperial Road from Susa to Sardis (black) and other highways (grey).

Here in detail:

To expand beyond the Tigris and Euphrates rivers (which form Mesopotamia, literally “between rivers”),  the Achaemenid Empire needed roads. This is why the first famous road in history is the Royal Road they built². Their system of rivers, roads, and canals allowed riders to go from one side of the empire to the other in about two weeks. That already stretched it. The empire couldn’t expand further and lost to the Greeks (the Achaemenid Empire suffered a pyrrhic victory in the famous Battle of Thermopylae, where the Greeks were led by the Spartans, and later lost in Marathon and Plataea³), and couldn’t maintain the control of Egypt for long either.

In summary, some kingdoms could easily overpower their neighbors. But what made them into empires was the speed of transportation. The Roman, Egyptian, and Persian empires were limited not by distance, but by speed of transportation across rivers, seas, and over land. In all cases, the limit of the size of their empires seems to have been between two weeks and a month of transport from the capital⁴. 

Rivers, Seas, and Roads

If this is true, most empires must have been built on the back of transportation systems. The more efficient the system, the bigger the empire could be.

Rivers and Canals

We’ve talked in the past about how the first civilizations in history flourished in river valleys.

We discussed the value of rivers for irrigation and trade. But we didn’t make it explicit that the rivers created political units because they allowed information, food, and armies to travel fast. If it took more than two weeks to get somewhere, empires couldn’t control these places: They could grow apart culturally, get overcome by a local independentist politician, be subject to revolt due to lack of local control or food, or simply get taken over by a neighbor.

So early kingdoms that emerged from rivers controlled a section of them. The speed of river transportation didn’t allow them to control their entirety.

We’ve talked about how Egypt was an outlier, mainly because of the Nile’s incredible seclusion.

Rivers were nevertheless much faster for travel than over land, which is why canals were so important, especially in China.

Source

China has two massive rivers close to each other, the Yellow and Yangtze. But as long as they weren’t connected, their valleys could not be controlled by one single group, and war would remain the status quo. That’s why the Chinese started the Grand Canal System in the 5th century BC, when Rome was still in its infancy.

To this day, many empires are built around river systems. For example:

• Pakistan is built around the Indus.

• The majority of India is built around the Ganges.

• West Germany and the Netherlands are built around the Rhine.

• London (and southern England) is built around the Thames.

• Burma is built around the Irrawaddy.

• Egypt is built around the Nile.

• France is built around the river system of the Seine, Rhône, Loire, and Garonne.

• The heartland of the US is built around the Mississippi.

• The American Northeast—of Canada and the US— is built around the Great Lakes and their canals.

Canals in northeast North America. Source.

Rivers are the oldest highways, but they weren’t the most efficient. It just took a bit of time to control a much faster one: the seas. But once humans had the technology, they forged ahead.

Thalassocracies

Phoenicia’s spread through the Mediterranean

It’s no coincidence that it was Phoenicia—approximately modern-day Lebanon—where seafaring appeared on the Mediterranean. Older civilizations emerged in Egypt and Mesopotamia and expanded around. Those who reached the Levant brought civilization with them but had no major river, lots of hills and mountains that made a focus on agriculture difficult, deep waters (caused by the same forces that create the hills), and lots of cedar trees covering the hills, which could be converted into boats. So they were naturally well positioned to focus on the sea.

Phoenicians brought Middle-Eastern civilization—and seafaring—to other parts of the Mediterranean. From their influence come the Greek, Carthaginian, and eventually Roman civilizations. 

Romans improved seafaring technologies. Without these advances, they couldn’t have unified the coasts of the Mediterranean into one single empire. In other words: Without boats, Europe would have taken much longer to develop, and Romance languages would probably not be spoken today.

The impact of sailing on the world progressed as technology improved. As we saw in A Brief History of Portugal, new sailing technologies allowed the Portuguese to navigate against the wind in the Atlantic, which allowed them to discover a path to the Indies rounding Africa, and to create its empire. The same is true of the Spanish empire. No new sailing techniques, no conquest of America.​​

The Portuguese Caravel. Source.

Astrolabe, carracks, cartographic advances… With every new technology came new and more powerful ways to sail the ocean. With the Industrial Revolution, the pace of innovation accelerated, and with it the transformations of human life: steamboats, submarines, aircraft carriers…

One of the things that might be hard to grasp is how massive the impact was in trade.

Source: US Department of Transportation

Even to this day, water transport costs a fraction of road transport. Imagine the difference when there were no roads! Transport over land was so expensive that most trade was simply impossible: The transport was more expensive than people were willing to pay.

And so this is how the seas allowed a different type of political organization: thalassocracies. As I mentioned in the article about Portugal, dozens of the most important civilizations in the world, from the Greek to the Majapahit, were first and foremost based on the seas:

• Carthage, Rome (including the Eastern Roman Empire), Greece, Venice, Genoa, and Aragon were based around the Mediterranean or parts of it.

• The Vikings and the Hanseatic League were based in the North and Baltic seas.

• The Chola Empire, Majapahit, or Srivijaya were based in the Indian Ocean.

• The Portuguese, Spanish, French, and British empires were based primarily around the Atlantic Ocean, but also the Indian Ocean.

In all these cases, thalassocracies started small, but as technology allowed people to move faster, the world shrunk. The distances covered in a few weeks increased, and the size of empires grew. No wonder the British were very eager to open the Suez Canal: It cut the distance to India, the jewel of the crown, making it that much easier to control.

Roadrunners

The same thing was true over land. 

We saw that the Persian empire had horsemen covering as much as 500 km per day in relays. Rome had 90,000 km of superbly paved roads for the horses and carriages of its messengers. The Inca empire had 40,000 km of unpaved roads for its chasqui runners who could go in 14 days from the capital Cuzco to the final frontier. These empires based on land needed the roads, without which the empires fell apart when they extended their borders beyond a couple of weeks of travel time.

This also helps explain for example why feudalism was so prevalent in Europe during the Middle Ages. Without well-maintained Roman roads, the time needed to go from one place to another extended, which made it that much harder to control big empires. Small, local warlords emerged. They were the ones with enough money to afford a horse and armor⁵.

It’s one of the reasons why the Frankish empire of Charlemagne couldn’t be united for long⁶, and why during that Medieval European time, the stronger polities in Europe were thalassocracies like Venice, Genoa, the Hanseatic League, the Vikings, or the Eastern Roman Empire.

In all these cases, the extension of the country depended on the speed of transportation.
So why were there countries that expanded beyond these limits?

Exceptions

Alexander the Great conquered most of the “known world” of his day.

Source

But his conquests took them 10 years, and after his death, the empire was split in six.

Source

Which further proves that a strong army, without proper transportation technologies, can’t hold an empire together.

Two thousand years later, Napoleon would suffer the same fate. 

The First French Empire at its greatest extent in 1812. Dark green: directly administered regions. Light green: client states. Source.

But he went all the way to Moscow, and that was too far away. The forces going there are in beige, the returning ones in black:

The Mongols conquered half the world. This was much easier because of horses and the ease of movement across the Eurasian Steppe, which shrunk distances.

But the minute its leaders died, it split.

Three centuries later, Russia would conquer everything from Moscow to the Pacific.

Source: Vivid Maps. If you want to see a video equivalent, you can find it here (1:30m on 2x speed, start at 30s).

But with a big difference: They only conquered the parts that were inhospitable, which means they couldn’t grow a local population that would fight back⁷.

Even then, the moment the Japanese became a threat, at the end of the 1800s, Russia hurried up the construction of the Trans-Siberian Railway: This vital artery was completed the same year as the war between the two countries exploded.

When the Czar wanted to reaffirm the belonging of Vladivostok to the Russian empire in front of the nervous emergence of Japanese power, he built the very expensive Transiberian military railway to send messengers there in just 14 days. Alaska was hopeless to reach in 14 days, so the Czar had sold it to the United States.—Quantitative Dynamics of Human Empires, Marchetti, Ausubel.

Another example of a country that overreached is Portugal—which controlled land across Europe, Africa, America, and Asia, and was the first to lose most of it.

You can also throw in England with its American colonies, or Spain with Latin America⁸. And in the US, the Union won against the Confederates, among other reasons because of its superior railroad system. 

Source

Empire Size and Transportation Speed

You can see this relationship between empire size and technological transportation speed in China, for example:

Source

I looked at the sizes of empires over time, and you can clearly see this evolution.

You can see that as time went by, the size of empires grew exponentially. So much so that if you compress the size (the vertical axis, with a logarithm), you get this graph:

The definition of an empire depends on who defines it. At the bottom right, you can see small empires. These were either pre-modern empires (in parts of the world disconnected from Eurasia, like the Aztecs), or they stretch the definition of empire, like the Bulgarian or the Oyo Empires. If instead you look at the top empires, the progression is unmistakable. 

We can clearly see the trend of empires getting exponentially bigger over time⁹.

Conclusion

Countries expand at the speed of transport technology. 

Why is this so important? Because the industrial revolution destroyed these realities. Railroads and steamboats exploded the distance that could be covered in a couple of weeks. Telegraphs made communication instantaneous. That means that the area that governments could control expanded. Suddenly, the potential area of influence of different countries started to overlap dramatically, and they fought for it.

Initially, Europeans fought for Africa, because it contained territory that hadn’t been conquered already. But after that, they just killed each other. Airplanes, the combustion engine, cars, the telephone… All these technologies that shrank the world, making anything in the world reachable in less than a couple of weeks, also allowed every country to control any other country, if only it could exert the power to do so.

And so this is how transportation technologies grew and killed empires. First, they helped empires grow larger by allowing them to expand their control. But by expanding their potential control to infinity, suddenly these transportation technologies became irrelevant, and other forces took over: nationalism, fertility, economic strength, cultural strength… All became the relevant determinant of politics after the Industrial Revolution because transportation technologies stopped mattering as much.

The Internet is having the same influence on the world. It’s eliminating the power of local governments and cultural nationalism. How will politics change as a result?

If you enjoyed this, read parts 1, 2, 3, and 4 and subscribe to get the next installments.

1. The capital of Egypt has always been at the beginning of the delta.

2. They also built a canal from the Nile to the Red Sea.

3. Thank you Gerardo for correcting me here when I said that the Greeks won in the Thermopylae.

4. Romans could have expanded more than others because they had fewer enemies: nobody to the south (the Sahara), the west (the Atlantic) or the north (the North Sea).

5. Which required the control of the resources of a small area.

6. Obviously the practice of splitting empires among children didn’t help, although these were frequently reunited through wars. They just couldn’t stay this way: Distances were too big.

7. Siberia couldn’t hold a population of any significant size because temperatures are too harsh and it has no river flowing south (the rivers flowing north freeze, making transportation impossible). The only southbound river in all of Russia is the Volga. It’s not surprising that it flows through the heartland of Russia.

8. This one took longer, but it’s proportional to the gap in technology that local forces had when the Spaniards invaded. 

9. This graph doesn’t consider transportation technologies.