Two thousand years ago, the Roman Empire was spreading across the world. This empire, which hosted tens of millions of people, extended from the Atlantic Ocean to North Africa, Asia, and Mesopotamia. However, the Roman Empire was not the largest empire in history — certainly not.

Tataryn/Wikimedia CommonsThe Roman Empire had a vast territory at its peak, but it was not the largest empire in history.

In fact, the Roman Empire was not even one of the top 10 largest empires in the past. Many other civilizations controlled more land and larger populations.

For example, in the 13th century, Genghis Khan and his descendants conquered the largest contiguous territories in history. By the early 1920s, the British Empire covered more than 25% of the Earth's surface.

So, which was the largest empire in history? Depending on how you measure it, there are multiple answers.

The Largest Empires in History by Land Area or Population Percentage

When it comes to the largest empire in history by land area — that is, the entire empire being contiguous — there is a clear answer: the Mongol Empire, which ruled a large part of Asia from 1206 to 1368.

Founded by Genghis Khan, the Mongol Empire quickly spread across Eurasia. Ultimately, it encompassed all of China and Mongolia, as well as parts of Korea, Iran, Iraq, Pakistan, Afghanistan, Uzbekistan, Armenia, Romania, Georgia, Ukraine, and Russia. At its peak, it covered an area of over 9 million square miles — nearly three times the size of the United States.

The decline of the Mongol Empire began with Genghis's death in 1227 but left a deep mark on world history. Genghis had countless children with his wives and concubines, and some scientists believe that one in every 200 men is a descendant of this ruler.

Zazaa Mongolia/Wikimedia CommonsA giant statue of Genghis Khan in Mongolia.

However, the Mongol Empire was not the largest empire in history in terms of global population percentage.

This title, according to some sources, belongs to the Achaemenid Empire, also known as the Persian Empire. It lasted from around 550 BC to 330 BC and covered a large part of present-day Iran, Egypt, and Turkey, as well as parts of Afghanistan and Pakistan. At its peak, approximately 44% of the world's population was governed by the Achaemenid Empire.

However, the Qing dynasty is also frequently listed as the largest empire in history by population percentage. Established in 1644, this dynasty lasted until 1912 and is believed to have represented about 40% of the world population at its peak. The global population during the Qing dynasty was also much larger, which technically means it housed more people than the Achaemenid Empire.

However, there is no confusion when it comes to the largest empire in history. This title belongs to the British Empire.

The Largest Empire in History: The British Empire

The largest empire in history, by total size, is the British Empire. The saying "the sun never sets on the British Empire" arose for this reason.

During the Age of Discovery, which took place between the 15th and 17th centuries, Britain and other European nations began to explore and colonize the world increasingly. The British Empire may have begun with explorer John Cabot's arrival in North America in 1497, commissioned by King Henry VII to search for unknown territories.

Wellcome CollectionThe cover of a sailors' hospital brochure from the 1900s, showing the reach of the British Empire.

After this, the British Empire continued to expand. Over the next few centuries, it controlled territories in North America, Oceania, Asia, and Africa, and also included parts of Central and South America. Egypt, India, Iraq, Fiji, and many more modern countries were under British rule. In fact, according to Stuart Laycock's The Countries We Have Invaded: And the Few We Never Got Around To, during the height of British power, there were only 22 countries in the world that Britain had not attempted to invade.

Indeed, at its peak, the British Empire stood out as the largest empire in history. In the 1920s, it controlled an area of over 13.7 million square miles and governed more than 400 million people. In other words, the British Empire covered about a quarter of the Earth's land surface and included approximately 23% of the global population.

However, like all empires, the British Empire would not last forever. After World War II, as nationalism spread worldwide, many countries under British rule demanded self-governance. India gained independence in 1947, followed by many other countries. However, Britain still retains a few "overseas" territories like Bermuda, and many countries that were once under British rule are now part of the British Commonwealth.

The British Empire came to an end by the close of the 20th century; some mark the official end with the handover of Hong Kong to China in 1997.

Other Civilizations That Reached Massive Sizes

While the British Empire and the Mongol Empire became the largest empires in history, the Qing dynasty and the Achaemenid Empire encompassed the largest percentage of the global population, and throughout history, other civilizations have also had large and influential empires.

For example, the Russian Empire, which lasted from 1721 until the Russian Revolution in 1917, is considered one of the largest empires in history. Spanning from the Black Sea to the Bering Strait, this empire covered nearly nine million square miles and was home to about 125 million people. Today, Russia is still the largest country in the world.

Public DomainMap of the Russian Empire in 1912.

Meanwhile, the Spanish Empire, which lasted from 1492 to the 1970s, conquered lands in Africa and the Americas and reached a size of five million square miles. However, due to having a population of under 20 million, the Spanish Empire was not as large as the Russian or British Empires.

In contrast, the Roman Empire covered only two million square miles at its peak and had a maximum population somewhere between 45 and 70 million. While it left its mark on world history in other ways, Rome was much smaller than the subsequent global powers.

Indeed, there is a clear answer to the question of the largest empire in history. While it can be evaluated in many ways, the British Empire ultimately became the empire that covered the most land on Earth. In the days when Britain controlled about a quarter of the Earth's surface, the sun truly never set on the British Empire.