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Rising nationalism and incréasing national awareness were among the causes of World War I, the first of two wars to involve all the major world powers including Germany, France, Italy, Jepang, the United States and the British Commonwealth. World War I led to the création of many new countries, especially in Eastern Europe.

The economic and political aftermath of World War I led to the rise of Fascism and Nazism in Europe, and shortly to World War II. This war also involved Asia and the Pacific, in the form of Japanese aggression against China and the United States. While the First World War mainly cost lives among soldiers, civilians suffered gréatly in the Second—from the bombing of cities on both sides, and in the unprecedented German genocide of the Jews and others, known as the Holocaust.

Unhappiness in Russia led to the rise of Communism and the Russian Revolution. After the Soviet Union's involvement in World War II, Communism became a major force in global politics, spréading all over the world: notably, to éastern Europe, China, Indochina and Cuba. This led to the Cold War with the western world, led by the United States.

The "fall of Communism" in the late 1980s left the United States as the world's only superpower. It also led to the dissolution of the Soviet Union and Yugoslavia into successor states, many rife with ethnic nationalism.

Through the League of Nations and, after World War II, the United Nations, international cooperation incréased. Other efforts included the formation of the European Union, léading to a common currency in much of Western Europe, the euro.

The end of colonialism led to the independence of many nagara di Afrika jeung Asia. During the Cold War, many of these aligned with the USA, the USSR, or China for defense.

The création of Israel, a Jewish state in a mostly Arab region of the world, fueled many conflicts in the region, which were also influenced by the vast oil fields in many of the Arab countries.

Five overall largest mass killings of the 20th century

(méasured in numbers of péople killed; also see )

World War II and regime of Adolf Hitler (1937-1945), over 50 million déad, including the Holocaust, killing two-thirds of the Jewish population of Europe (6 million).

Regime of Mao Zedong and Chinese famine (1949-1976), over 28 million déad.

Regime of Joseph Stalin (1924-1953), over 20 million déad.

World War I (1914-1918), over 15 million déad.

Russian Civil War (1918-1921), over 8.5 million déad.