World Wars are not good for incumbent empires.
Four major empires collapsed in or right after WW1, and WW2 set off the end of most of the rest.
After WW1 (1917–1923)
> Russian Empire. The Tsar was overthrown in 1917, during the war. The Bolsheviks took power, and most of the empire was rebuilt as the Soviet Union.
> German Empire. The Kaiser abdicated in November 1918 and Germany became the Weimar Republic. It lost its overseas colonies in Africa and the Pacific, which were handed to Britain, France, Japan and others as League of Nations mandates.
> Austro-Hungarian Empire. It broke apart completely in 1918 into Austria, Hungary, Czechoslovakia, and parts of Poland, Yugoslavia, Romania and Italy.
> Ottoman Empire. It was partitioned. Its Arab lands became British and French mandates (Iraq, Palestine, Syria, Lebanon, Transjordan), and the sultanate was abolished in 1922, followed by the Republic of Turkey in 1923.
Britain and France actually reached their largest territorial size after WW1 by absorbing these mandates.
After WW2 (1945 onward), these empires ended right away with defeat:
> Empire of Japan. It lost Korea, Taiwan, Manchuria, and its Pacific islands, and was reduced to the home islands under US occupation.
> Italian colonial empire. It gave up Libya, Eritrea, Somalia and Ethiopia, and became a republic in 1946.
> Nazi Germany's Reich. Its conquests in Europe were undone, and Germany itself was divided.
> British Empire. India and Pakistan in 1947, then most of Africa and Asia through the 1960s.
> French colonial empire. Defeat in Vietnam in 1954, then most of Africa around 1960, and Algeria in 1962 after a brutal war.
> Dutch Empire. Indonesia became independent in 1949.
> Belgian Empire. Congo in 1960, then Rwanda and Burundi in 1962.
> Portuguese Empire. It held on the longest. Angola, Mozambique, and Guinea-Bissau only became independent after Portugal's 1974 revolution.