The Korean War has been called ''the Forgotten War'' in the United States, where coverage of the 1950s conflict was censored and its memory decades later is often overshadowed by World War II and the Vietnam War.
But the three-year conflict in Korea, which pitted communist and capitalist forces against each other, set the stage for decades of tension among North Korea, South Korea and the United States.
It also helped set the tone for Soviet-American rivalry during the Cold War, profoundly shaping the world we live in today, historians said.
As tensions between North Korea and the United States continue to mount amid missile tests and l taunts, here is a brief guide to the Korean War and the impacts that linger more than 60 years after its end.
How did the Korean War start?
The Korean War began when North Korean troops pushed into South Korea on June 25, 1950, and it lasted until 1953. But experts said the military conflict could not be properly understood without considering its historical context.
Korea, a Japanese colony from 1910 until 1945, was occupied by the United States and the Soviet Union at the end of World War II. The United States proposed temporarily dividing the country along the 38th Parallel as a way to maintain its influence on the peninsula, which bordered Russia, said Charles K. Armstrong, a professor of Korean history at Columbia University.
''A divided Korea was something unprecedented,'' he said.
But the divide lasted in part because of competing visions among Koreans for the country's future. ''Fundamentally it was a civil war, fought over issues going back into Korea's colonial experience,'' said Bruce Cumings, a professor of history at the University of Chicago.
[Video: Combat footage, Korean war -- newsreel (1950) Watch on YouTube.]
In 1948, the American-backed, anti-communist southern administration, based in Seoul, declared itself the Republic of Korea. It was led by Syngman Rhee, who lived in exile in the United States for many years and was installed as the South Korean leader by the Office of Strategic Services, a predecessor to the Central Intelligence Agency, Professor Cumings said.
Soon after, the Soviet-backed, communist northern administration, based in Pyongyang, declared itself the Democratic People's Republic of Korea. Its leader was Kim Il-sung, who fought alongside communist forces during the Chinese civil war and was the grandfather of North Korea's current dictator, Kim Jong-un.
Each regime was unstable, rejected the legitimacy of the other and considered itself to be Korea's sole rightful ruler. Border skirmishes between the two were frequent before the Korean War began.
Who were the combatants?
The war pitted South Korea and the United States, fighting under the auspices of the United Nations, against North Korea and China.
Other nations contributed troops, too, but American forces did most of the fighting. ''The South Korean Army virtually collapsed'' at the start of the war, Professor Cumings said.
The Soviet Union supported North Korea at the beginning of the war, giving it arms, tanks and strategic advice. But China soon emerged as its most important ally, sending soldiers to fight in Korea as a way to keep the conflict away from its border.
The Chinese leader, Mao Zedong, also saw China's participation in the war as a way to thank Korean Communists who fought in the Chinese civil war, Professor Cumings said.
''There was a lot of field contact between American and Chinese forces,'' Professor Armstrong said. ''In a sense, this was the first and only war between China and the United States, so far.''
How damaging was it?
The war devastated Korea. Historians said that between three million and four million people were killed, although firm figures have never been produced, particularly by the North Korean government. As many as 70 percent of the dead may have been civilians.
Destruction was particularly acute in the North, which was subjected to years of American bombing, including with napalm. Roughly 25 percent of its prewar population was killed, Professor Cumings said, and many of the survivors lived underground by the war's end.
''North Korea was flattened,'' he said. ''The North Koreans see the American bombing as a Holocaust, and every child is taught about it.''
Damage was also widespread in South Korea, where Seoul changed hands four times. But most combat took place in the northern or central parts of the peninsula around the current Demilitarized Zone, which divides the countries, Professor Cumings said.
How did it end?
Technically, the Korean War did not end.
The fighting stopped when North Korea, China and the United States reached an armistice in 1953. But South Korea did not agree to the armistice, and no formal peace treaty was ever signed.
''There is still a technical state of war between the combatants,'' Professor Cumings said.
Neither North nor South Korea had achieved its goal: the destruction of the opposing regime and reunification of the divided peninsula.
Since 1953 there has been an uneasy coexistence between North and South Korea, which hosts over 20,000 American troops. At one time hundreds of American nuclear weapons were based there.
''It was from the Korean War onward that we had a permanent, global American military presence that we had never had before,'' Professor Armstrong said. Other countries that host American troops include Qatar, Japan, Italy and Germany. ''It was a real turning point for America's global role.''
In the decades after the war, South Korea transformed into an economic powerhouse. Professor Cumings said many of its citizens now know little about the conflict and have ''a fatalistic orientation'' toward the economically isolated North.
Meanwhile, North Korea became ''the world's most amazing garrison state with the fourth largest army in the world.''
''Its generals are still fighting the war,'' Professor Cumings said. ''For them it has never ended.''