Common causes

One side's defense looks like a threat to the other

Land, water, oil and trade routes

Political, religious or national beliefs

Leaders misjudge the other side's resolve

Twentieth-century conflicts

Conflict	Years	Estimated deaths
World War I	1914–1918	About 15–20 million
World War II	1939–1945	About 70–85 million
Korean War	1950–1953	About 3 million
Cold War	1947–1991	Proxy wars; no direct U.S.–Soviet war

Estimates vary widely among historians.

The Cuban Missile Crisis brought the world close to nuclear war before both sides stepped back.

Sources: Encyclopaedia Britannica; National WWII Museum; United Nations

Wars usually come from fear, competition and miscalculation together; deterrence and diplomacy work to prevent them.