1950: North Korean invasion of South Korea.
Excerpted from History.com:
On 25 June 1950, some 75,000 soldiers from the North Korean People’s Army poured across the 38th parallel, the boundary between the Soviet-backed Democratic People’s Republic of Korea to the north and the pro-Western Republic of Korea to the south. This invasion was the first military action of the Cold War. By July, American troops had entered the war on South Korea’s behalf. As far as American officials were concerned, it was a war against the forces of international communism itself. After some early back-and-forth across the 38th parallel, the fighting stalled and casualties mounted with nothing to show for them. Meanwhile, American officials worked anxiously to fashion some sort of armistice with the North Koreans. The alternative, they feared, would be a wider war with Russia and China–or even, as some warned, World War III. Finally, in July 1953, the Korean War came to an end. In all, some 5 million soldiers and civilians lost their lives during the war. The Korean peninsula is still divided today.