Comparing empires by peak land area
The British Empire is generally cited as the largest in history by territory, covering roughly 35.5 million square kilometers around 1920, about a quarter of the world's land. The Mongol Empire, peaking in the late 13th century, was the largest contiguous land empire at around 24 million square kilometers. The Russian Empire reached about 22.8 million, the Spanish Empire around 13 million at its 18th-century height, and the Roman Empire roughly 5 million at its peak under Trajan.
Rise-and-fall trajectories
Each empire follows a distinct shape over time. The Mongol Empire expanded explosively within a single lifetime then fragmented into khanates within decades. Rome rose slowly over centuries and declined gradually. The British and Spanish maritime empires grew through overseas colonies and contracted sharply during 20th-century decolonization. Plotting peak extent against duration shows that contiguous land empires often expanded faster but fragmented sooner than dispersed maritime ones.