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The immediate trigger for the fall of the Western Roman Empire was the sack of Rome by barbarian groups such as the Visigoths (410 AD) and Vandals (455 AD), culminating in the deposition of the last emperor, Romulus Augustulus, by Odoacer in 476 AD. Root causes included internal political instability, economic decline, military defeats, overreliance on mercenary forces, and pressures from migrating and invading peoples at the empire’s borders.

Brexit was triggered by the 2016 referendum in which a majority of UK voters chose to leave the European Union. The root causes included longstanding concerns over national sovereignty, immigration, economic policy, and skepticism about EU regulations and political integration.

The French Revolution was triggered by the financial crisis caused by government debt and food shortages, culminating in the storming of the Bastille on July 14, 1789; its root causes included social inequality among the estates, Enlightenment ideals, and dissatisfaction with absolute monarchy.

The Great Depression was triggered by the stock market crash of October 1929, but its root causes included over-speculation in the stock market, widespread bank failures, unequal distribution of wealth, agricultural overproduction, and weaknesses in the international financial system.

The Cold War began after World War II, triggered by growing tensions between the United States and the Soviet Union over the division of Europe and the future of Germany. The root causes included ideological differences between capitalism and communism, mutual distrust, and competing ambitions for global influence.

The immediate trigger for the collapse of the Soviet Union was the failed August 1991 coup by hardline Communist officials, which weakened central authority and accelerated republics declaring independence. Root causes included long-term economic stagnation, political corruption, nationalist movements, the costly arms race with the West, and reforms like Perestroika and Glasnost that undermined the Communist Party's control.

The immediate trigger for U.S. entry into World War II was the Japanese attack on Pearl Harbor on December 7, 1941, but root causes included escalating conflicts with Axis powers, economic sanctions on Japan, and a desire to support allies threatened by Nazi expansion.

The Protestant Reformation was sparked in 1517 when Martin Luther posted his Ninety-Five Theses criticizing the Catholic Church’s sale of indulgences; its root causes included widespread dissatisfaction with church corruption, the rise of Renaissance humanism, and growing calls for religious and political reform across Europe.

The immediate cause of the Cuban Missile Crisis was the discovery in October 1962 of Soviet missiles being installed in Cuba, just 90 miles from the U.S. coast. Root causes included Cold War tensions, the U.S. placement of missiles in Turkey, and the failed Bay of Pigs invasion, which pushed Cuba closer to the Soviet Union for protection.