ARCHIVAL (ASSOCIATED PRESS, 1953):NEWS REEL: South Korean villages awoke to a world suddenly filled with noise and flame. The Communists had finally launched their undeclared, all-out war of conquest!

NARRATION: In the summer of 1950, the U.S. watched with alarm as North Korean Communists invaded South Korea.

ARCHIVAL (GETTY, 1951):NEWS REEL: Thousands of civilian refugees clogged the roads, fleeing the invading Reds!

NARRATION: The move seemed to confirm what President Harry Truman had long feared: communism, already dominant in the Soviet Union and China, might one day encircle the globe.

ARCHIVAL (GETTY, 7-19-50):PRESIDENT HARRY TRUMAN: We are united in detesting Communist slavery. We know that the cost of freedom is high, but we are determined to preserve our freedom, no matter what the cost.

NARRATION: But Truman faced a problem when he decided to rush U.S. troops to stop the Communist advance in Korea: according to the Constitution, Congress was the only branch of government with the power to declare war.

MARY L. DUDZIAK (PROFESSOR OF LAW, EMORY UNIVERSITY): The president isnt the king who gets to decide to use force without anyone having a check on his power. because the power to go to war is so destructive.

NARRATION: Having Congress debate whether to declare war would take precious time though, and there was no guarantee what theyd decide. So Truman used a loophole offered by the newly-formed United Nations.

H.W. BRANDS (HISTORIAN, UNIVERSITY OF TEXAS): It just so happened that the United Nations was able to authorize member states to go to the aid of South Korea. The United States was going under the authorization at the U.N. to act essentially as a policeman.

NARRATION: Truman decided to take the gamble, skipping over Congress and Constitutional principle.

MARY L. DUDZIAK: And he was pressed, you know, was this a war? And Truman said, no, its a police action. He wanted to not use the war word because he hadnt asked for a declaration of war from Congress.

NARRATION: But for U.S. troops landing in Korea, who made up the vast majority of the U.N. force, Trumans clever wording didnt change anything.

EDWARD BROOKS (U.S. ARMY SERGEANT, KOREA): Somebodys shooting at you, and you gotta shoot back, its war, its not a police action.

NARRATION: And as the already war-weary troops pressed north, conditions became even more treacherous.

ARCHIVAL (GETTY):NEWS REEL: Frozen, mountainous countryside north of Seoul.

ARCHIVAL (ASSOCIATED PRESS, 1953):NEWS REEL: This far north, winter comes early to Korea. Marines are advancing over ground frozen harder every day.

NARRATION: Harry Heath and Jack Allen, who had advanced near the border with China, remember the unthinkable cold.

HARRY HEATH (U.S. MARINE, KOREA): It was forty degrees below zero there. When you got pinned down, your feet wouldnt be moving and they would freeze in there.

JACK ALLEN (U.S. MARINE, KOREA): We walk in this hospital. There were bodies, people laying down. Their hands were frozen. You could take some people and snap their fingers off, they were frozen so badly. Some of these guys had lost arms and fingers and toes.

NARRATION: And despite the fact that Congress hadnt declared it a war, Truman worried that the fighting in Korea could explode into a full-fledged World War III when the Chinese invaded.

JACK ALLEN: Must have been around 9 or 10 oclock at night, we could hear whistles and bells and drums and hollering and and it was the Chinese. And they came up and overran us.

ARCHIVAL (ASSOCIATED PRESS, 1953):NEWS REEL: 200,000 red Chinese, swarmed across the Yalu River and engaged the weary U.N. troops.

H.W. BRANDS: All of a sudden, this small war between two small countries inon the Korean Peninsula, South Korea and North Korea, had become a big war involving the worlds greatest power, the United States, and Communist China which had the worlds largest military. Truman, at that point, realized that he had gone farther than he wanted to go.

ARCHIVAL (GETTY, 1950):NEWS REEL: The heavily outnumbered U.S. fighting men were trapped in what looked like a hopeless situation.

HARRY HEATH: There was 15,000 Marines and Army and we were surrounded. But our motto was, uh, we cant win, we cant lose, but were not gonna give up.

ARCHIVAL (GETTY, 1950):NEWS REEL: In the mountainous terrain of North Korea, the breakout from their encircled position was a battle unparalleled in American military history.

JACK ALLEN: These Chinese, there were about four of five machine guns that would just cut them down. You could look down that mountain and you could see it was like cord wood of bodies.

This guy threw a concussion grenade, and it blew up. It hit me in my knee and I couldnt walk. The company commander said, Get all the guys that are wounded and get them out of here. Thats the closest I ever came to not surviving.

NARRATION: After being pushed back by the Chinese at the end of 1950, U.N. and South Korean troops kept fighting until a truce in 1953 ended three years of hostilities. In the end, neither side gained significant territory. Three to four million people had died, including nearly 40,000 Americans and an estimated two million civilians.

But troops coming home discovered that their experience in Korea, dubbed a mere police action by Truman, hadnt truly captured the publics interest.

HARRY HEATH: One of my friends said, Harry, where you been? I havent seen you in a long time. I said, Ive been in Korea! He said, Well wheres Korea?

I feel that the Korean War was an important war because it stopped the spread of communism and to forget it I think is wrong.

NARRATION: And in the decades since, Trumans decision to send troops to Korea without approval from Congress has had a lasting impact. Presidents of both parties have followed Trumans example, from George H.W. Bush sending troops into a civil war in Somalia, to Barack Obama ordering airstrikes in Libya.

H.W. BRANDS: Presidents after Truman realized that its very convenient not to have to bring Congress aboard because they asked difficult questions. This is something thats lost, when democracies go to war citizens need to be informed.