After Serbian officials assassinated Austria-Hungary's heir, Russia joined the war in Serbia's Defense, as well as calling its own allies France and Great Britain. After a secret German note to the Mexican government was discovered by the British asking Mexico to invade the USA, the United States joined the war and it ended a year later in victory.
War in Vietnam
They're in the trees!
After Germany invaded poland in 1939, Britain and France joined the war in Polands defense. With Germany's  new strategies they quickly expanded over all of Europe. Despite the United States wanting of neutrality, the Empire of Japan attacked pearl harbor and the US joined the war. The allies invaded from France and took control of germany but the war didnt end until 1945 when the USA bombed and destroyed Japan
Cold War
North Korea invaded South Korea in 1950 in an attempt to re unite the Korean peninsula with a Communist government. The US immediately sent troops to South Korea in its defense and to stop the spread of communism in Asia. They pushed back North Korea so far that China had joined the war on North Korea's side, but after three years the war ended in peace and the peninsula was still divided.
Gulf War I
North Vietnam attacked South Veitnam in 1955 to unite Vietnam with a communist style government. The US joined the war in 1964 when North Vietnam attacked a US warship. The war lasted for 20 years and when the US left the war in 1973, North Vietnam finally conquered South Vietnam in 1975.
The Cold War began in 1945 shortly after the end of world war two. It was between the United States and the Soviet Union in showing who was the more dominant Super Power of the world. The cold war lasted for a long time without any actual gunfire. It finally ended in 1991 after the collapse of the Soviet Union.
The United States joined the war with Kuwait against Iraq after Iraq invaded Kuwait. The war lasted from 1990 to 1991 with a victory in the end.