The Cold War turned hot in 1950, when communist North Korea invaded U.S. backed South Korea. More than 20 other countries, under a U.S.-led United Nations command, came to the defense of South Korea.
By all accounts, South Korea was totally unprepared in late June 1950, when tens-of-thousands of Russian-equipped North Korean troops attacked. It took the North's invasion force just a few weeks to push the South Korean and U.S. troops to Busan, in the far south of the peninsula.
The tide turned in mid-September when General Douglas MacArthur boldly sent divisions ashore at Inchon, behind North Korean lines.
The North Koreans retreated. Days later, Seoul was in the hands of UN troops. A month later, Pyongyang was captured.
At the end of World War II, the Korean Peninsula had been divided roughly along the 38th parallel, with the communist North allied with the Soviet Union, and later with communist-led China. The South was allied with the United States.
In late November, the Chinese, angry at what they saw as threatening U.S. activity along its border with North Korea, joined the fight with 300,000 troops. That vast force began to push the allies back south of the 38th parallel.
Mike O'Callaghan, the former governor of the U.S. state of Nevada, lost a leg as an army private in Korea. He says, despite the presence of so many Chinese on the battlefield, it was the North Koreans who were feared most.
"They were much more intense, and they were much more aggressive than the Chinese were," Mr. O'Callaghan said. "In other words, you could break contact with a Chinese patrol fairly easy. With the North Koreans you couldn't. They would keep coming."
It was the middle of 1951 before the Soviet Union proposed a truce. Talks started and stopped, and started again during the next two years, while a stalemate emerged on the battlefield.
The fighting ended when an armistice was signed on July 27, 1953, and both countries retreated to the original border.
By then, more than a million soldiers on both sides were reported dead or missing. Some estimates say as many as 3 million civilians died, millions more became refugees, and much of the peninsula's agriculture and industry was destroyed.
Hu Nam Sung teaches military strategy at South Korea's National Defense University. He says the armistice left a flawed peace.
"One of the greatest defects is that it had a fundamental limitation in that there was no way to impose punitive measures in case of willful violation by a party," he said. "During the past 50 years, North Korea violated [it] tens-of-thousands of times, but we could not deter that kind of violation."
Former South Korean presidential advisor Choong Nam Kim recalls that, when the armistice was signed, some elders accused the Americans of betraying Korea, because they left a divided peninsula.
"Korea had been a united country for a thousand years," he said. "And they believed that North Korea is occupied by Russian and Chinese communists. The allies came to Korea to save Korea. So, why they stopped the war at the same territory line after so much sacrifice?"
Some South Koreans fear younger generations have forgotten the war and its implications. Dr. Kim, now a fellow at the East-West Center in Hawaii, shares that concern.
"They have no idea how their parents' generation sacrificed, how they survived and how United States and other countries came to Korea to save [them] from the communist control. Their priority is reunification. No matter what happens, they want to reconcile with North Korea, they want to unify," Dr. Kim said.
Fifty-years later, North Korea and South Korea remain separated, but much has changed around them.
The Soviet Union has collapsed and no longer aids the impoverished North. The United States, China, and South Korea now are trading partners. What is more, they are cooperating to maintain peace on the Korean Peninsula.
But since October, when North Korea admitted having a secret nuclear program to U.S. officials, in violation of several international pledges, tensions have risen.
Despite international pressure, Pyongyang has abandoned its pledges to remain nuclear free, and has issued a stream of rhetoric, threatening to turn the peninsula into a "sea of fire."
It is those threats, say older South Koreans such as Professor Hu and Dr. Kim, that make it imperative to remember the Korean War.