World War I wasn’t just a conflict between nations—it was a war between empires. Western European empires like Great Britain and France had overseas colonies around the world, while eastern empires like Austria-Hungary and Russia ruled European and North Asian territories connected by land. The assassination of Archduke Franz Ferdinand on July 28, 1914, was itself an anti-imperialist murder, planned by members of Young Bosnia angry over Austria-Hungary’s annexation of Bosnia-Herzegovina.

European competition for imperial territories helped set the stage for the rivalries that played out during the First World War, and the war in turn had a major effect on the balance of imperial power. The Russian, German, Austria-Hungarian and Ottoman empires all collapsed during or shortly after the war, which ended with a treaty that ceded Germany’s overseas colonies to the victors.

By the time World War I began, almost all of the African continent was under some form of colonial rule by Great Britain, France, Germany, Italy, Belgium, Spain or Portugal. Most of this colonization happened after 1880, during a period known as the Scramble for Africa or the Partition of Africa, in which European empires competed with each other for control of African territories.

In the centuries before the Scramble for Africa, European empires had invaded African coastal nations to capture and enslave people but mostly hadn’t managed to invade farther inland due to navigational difficulties and the threat of diseases like malaria. After the legal abolition of slavery, new technologies like steamboats and quinine allowed Europeans to invade much more of the continent.

The European empires that invaded Africa saw colonization as a way to exploit forced labor, extract resources and become more powerful in relation to other European empires. Although colonialism in Africa wasn’t a direct cause of World War I, it helped create an environment in which European empires thought of themselves as rivals who could only succeed at the expense of other empires. For example, France and Germany, two main rivals during World War I, competed with each other for control of Morocco in the decade before the war.

“France and Germany did not go to war over Morocco,” says Richard Fogarty, a history professor at the University at Albany and co-editor of Empires in World War I: Shifting Frontiers and Imperial Dynamics in a Global Conflict.

“What happened, though, was that they were conditioned to think of each other as competitors,” he says, “and to think of the world as this zero-sum game in which the French pursuit of empire could only come at the expense of the German pursuit of empire.”

Great Britain was also concerned about Germany’s attempt to build a navy that might challenge its own. Although Germany was nowhere close to achieving this, Fogarty says, “the British couldn’t even tolerate the idea of a threat to their naval supremacy, because they had an empire to secure. And so that made them hypersensitive to any competition.”

French and British fears about Germany’s empire-building are part of what drove European nations to form alliances and informal agreements in the decades leading up to World War I, dividing Europe into roughly two opposing camps.

In contrast to most of the Western European empires, the Austria-Hungarian, Russian and the Ottoman empires were contiguous, with territories connected to each other by land. On the eve of World War I, the three empires’ borders converged at the Balkans—a region in southeastern Europe that the empires viewed as strategically valuable, and played a major role in the start of the Great War.

The Ottoman Empire had previously controlled much of the Balkans but lost most of its territory there in the 19th century. Austria-Hungary took advantage of the Ottoman Empire’s retreat by occupying Bosnia-Herzegovina, a region in the Balkans that the empire annexed in 1908.

It was this occupation and annexation that the revolutionary group Young Bosnia was protesting when it assassinated Archduke Franz Ferdinand, the presumptive heir to the Austria-Hungarian throne. After the assassination, Austria-Hungary accused neighboring Serbia, another nation in the Balkans, of assisting Young Bosnia and declared war on Serbia.

Russia ostensibly agreed to back Serbia against Austria-Hungary because it was a fellow Slavic state; but Andrew Jarboe, a history professor at Berklee College of Music who co-edited Empires in World War I with Fogarty, suggests Russia was also motivated by imperial interests in the Balkans.

“I really think Russia’s calculus is: if they don’t respond militarily, they render themselves obsolete in this region,” he says.

Russia fought World War I on the side of the Allies, which included Great Britain, France, Italy and Japan, but left the Great War in 1917 when revolution and civil war broke out in its own empire. This led to the collapse of the Russian Empire and the establishment of the Soviet Union.

The opposing side, the Central Powers, is where most of the other imperial collapses happened. The 1919 Treaty of Versailles dismantled the Austria-Hungarian Empire in Europe and the German Empire both in Europe and abroad. Great Britain, France and Belgium divided most of Germany’s African colonies among themselves, while Japan took over Germany’s colonies in China and the North Pacific. In addition, the treaty imposed measures on the Ottoman Empire that led to its dissolution in 1922.

When Adolf Hitler rose to power, he deliberately used the existence of a previous German Empire to justify his “Third Reich," or “Third Empire,” which he imagined would take control of Europe (in his mind, the Holy Roman Empire was the “First Reich”). When he invaded Poland in 1939, sparking World War II, part of his rationale that he was conquering territory that rightly belonged to Germany—an excuse that many imperialists had used before and would use again.