The years after World War II weren't peaceful. But they didn't erupt into World War III either (cross your fingers). For much of the time after World War II, the major world powers were preoccupied with a game of nuclear standoff.
The major powers, by the way, turned out to be the United States and the Soviet Union. The United States expected to enjoy its nuclear monopoly for 20 years or more, but the Soviets surprised everyone by developing their own atomic bomb in 1949. Allies on the winning side of World War II, the nations became bitter rivals very soon afterward.
Soviet foreign policy, reflecting Josef Stalin's viciously paranoid behavior toward any rival — real or imagined, internal or abroad — became increasingly exclusionary and closed off. Soviet goals included maintaining control over satellite communist states, several set up in Soviet-controlled Eastern Europe in the wake of World War II, while keeping out foreign cultural and economic influences.
The United States emerged as leader of the West — meaning western Europe, the Western Hemisphere, and developed nations anywhere that resisted communism and promoted (or at least permitted) the private pursuit of profit in their trade policies.
Daring each other to blink
With their nuclear arsenals, the Soviet Union and United States engaged in a Cold War. It amounted to a diplomatic, cultural, political, and military standoff.
In diplomatic and military terms, the Cold War took the form of each side daring the other to fire the first nuclear shot. Both nations built more and more, bigger and bigger missiles and warheads. Missiles became capable of delivering a nuclear bomb from a Nebraska wheat field into downtown Moscow. Both nations developed the ludicrously tragic ability to blow up the Earth several times over.
This madness was tempered a little with a Nuclear Test Ban Treaty in 1963, numerous arms talks, and even arms reduction agreements, but the two nations basically kept their guns pointed at each other's heads until one, the economically ruined Soviet Union, blinked — or in this case, fell apart. Along the way, several other countries built nuclear arsenals — China prominent among them.
Returning to arms
Meanwhile, many regional wars raged. Among them, the United States was embarrassed in a futile attempt to keep Vietnam, a former French colony (and before that, a sometime Chinese vassal state) in Southeast Asia, from going communist. The Soviets squandered a lot of resources and international good will fighting Muslim rebels in Afghanistan.
When Israel, a new Jewish state, was established in 1948 in what was British-ruled Palestine, surrounding Arab nations joined Palestinian Arabs in opposing it. The disagreement turned violent many times, with wars in 1956, 1967, 1973, and 1982. Also in the region, Iraq fought Iran. Then Iraq invaded Kuwait and a U.S.-led international force turned it back.
Horrible intertribal violence broke out in Africa. Terrorist bombings threaten people on every continent. Clearly, humanity has not come close to achieving a world without war.