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Introduction World War I (1914–1918) Causes of WWI Effects of WWI World War II (1939–1945) Causes of WWII Effects of WWII The Korean War (1950–1953) The Vietnam War (1955–1975) What These Wars Changed ConclusionThe 20th century was the most violent in human history. Two world wars, fought between 1914 and 1945, killed an estimated 70 to 85 million people combined. Other major conflicts — in Korea, Vietnam, the Middle East, and elsewhere — added tens of millions more. What made this century different was not simply the number of people who died, but how they died: through industrialized warfare, aerial bombing of civilian areas, deliberate famine, and genocide. To understand these wars, you need to understand what caused them and what they changed.
World War I did not begin because of a single event, even though the assassination of Archduke Franz Ferdinand of Austria on June 28, 1914 is usually described as its trigger. The real causes had been building for decades.
When Franz Ferdinand was killed by Gavrilo Princip, a Bosnian Serb nationalist, Austria-Hungary issued an ultimatum to Serbia. Russia mobilized to support Serbia. Germany declared war on Russia. France came in. Germany invaded Belgium. Britain declared war on Germany. Within weeks, the entire continent was at war.
Politically, the war ended four empires: the German, Austro-Hungarian, Ottoman, and Russian. The Treaty of Versailles (1919) placed full blame on Germany, imposed reparations of 132 billion gold marks, stripped Germany of territory, and limited its military. These terms created economic hardship and national humiliation in Germany that directly fed the conditions for World War II.
The causes of World War II are closely connected to how World War I ended. The Versailles settlement was neither fair enough to win German acceptance nor strong enough to prevent German rearmament. When the Great Depression hit in 1929, Germany's economy collapsed, and the Nazi Party rose to power.
Adolf Hitler became Chancellor in January 1933. He began rearming the country, reoccupied the Rhineland (1936), annexed Austria (1938), and took Czechoslovakia's Sudetenland (1938). Britain and France pursued appeasement. When Hitler invaded Poland on September 1, 1939, Britain and France finally declared war.
In Asia, Japan had been pursuing imperial expansion since its invasion of Manchuria (1931) and China (1937). Japan's attack on Pearl Harbor (December 7, 1941) brought the United States into the war.
The war produced the atomic bombings of Hiroshima and Nagasaki (August 1945), introducing nuclear weapons as a permanent feature of international politics.
Politically: Ended European colonial dominance; US and Soviet Union emerged as superpowers. United Nations founded (1945). Nuremberg trials established that state leaders could be held personally accountable for war crimes. State of Israel established (1948).
Korea was divided along the 38th parallel after World War II, with Soviet-backed communists in the north and a US-backed government in the south. When North Korea invaded the south in June 1950, the United States led a UN coalition to push the North back. China intervened in October 1950. The war ended in 1953 in an armistice, not a peace treaty, leaving the peninsula divided exactly as before. Approximately 3 million people had died.
After France was defeated at the Battle of Dien Bien Phu (1954), Vietnam was divided into communist North and US-backed South. American involvement escalated through the 1960s, reaching over 500,000 US troops by 1968. The war killed an estimated 3.5 million Vietnamese and approximately 58,000 US military personnel. North Vietnam reunified the country under communist rule in 1975, demonstrating the limits of American military power.
The post-1945 world was built in direct response to what these wars had done — an attempt to build a framework that would prevent a repeat of the previous thirty years.
The wars of the 20th century were not simply larger versions of what had come before. They brought together industrial technology, extreme ideology, and mass mobilization in ways that made them qualitatively different from earlier conflicts. The causes — nationalism, economic crisis, imperial rivalry, failed diplomacy — are comprehensible. What makes this history worth studying is not just the scale of destruction but the fact that much of it was preventable, and understanding where the decisions went wrong matters for how the world has tried to organize itself since.