context: A French Army unit is on patrol in pre-Vietnam IndoChina in July 1954 during the First Indochina War. The captain of the patrol curses the land when they see nothing. Then, the unit is suddenly ambushed by hundreds of Vietminh soldiers who kill the officers and, although the unit kills many Vietminh, it is eventually overrun. One Vietminh officer, Nguyen Huu An (Duong Don), hypothesizing that if they take no prisoners the French will eventually stop sending troops, orders the execution of all surviving French soldiers.Eleven years later, Lieutenant Colonel Hal Moore (Mel Gibson), a dedicated U.S. soldier, is deeply committed to training his troops, who are preparing to be sent to Vietnam. The night before their departure, the unit's officers hold a party to celebrate. Moore learns from a superior officer that his unit will be known as the 1st Battalion / 7th cavalry regiment. He is disquieted because the 7th Cavalry regiment was the unit commanded by General George Custer in the 19th Century when he and his men were slaughtered at the Battle of the Little Bighorn in 1876. Moore is also dismayed because President Lyndon B. Johnson has decreed that the war would be fought "on the cheap," without declaring it a national emergency. As a result, Moore believes he will be deprived of his oldest, best-trained soldiers (a formal declaration of war would have meant mobilization and extension of the terms of enlistment for volunteer soldiers) - about 25% of his battalion - just prior to shipping out for Vietnam. Before leaving for Vietnam, Moore delivers a touching speech to his unit:"Look around you, in the 7th Cavalry,we got a Captain from the Ukraine, another from Puerto Rico, we got Japanese, Chinese, Blacks, Hispanics, Cherokee Indian, Jews and Gentiles, all American. Now here in the States some men in this unit may experience discrimination because of race or creed, but for you and me now, all that is gone. We're moving into the valley of the shadow of death, where you will watch the back of the man next to you, as he will watch yours, and you won't care what color he is or by what name he calls God. Let us understand the situation we're goin' into battle against a tough and determined enemy. I can't promise you that I will bring you all home alive, but this I swear: When we go into battle, I will be the first one to set foot on the field, and I will be the last to step off. And I will leave no one behind. Dead, or alive, we all come home together. So help me God."After arriving in Vietnam in November 1965, he learns that an American base has been attacked, and is ordered to take his 395 men after the enemy and eliminate them, despite the fact that intelligence has no idea of the number of enemy troops. He leads a newly created air cavalry unit into the Ia Drang Valley against over 4,000 well equipped enemy soldiers.An emotional toll is taken back home, where Moore's wife Julie (Madeleine Stowe) and another soldier's wife take over the job of delivering telegrams that inform families (mainly wives like themselves) living at Fort Benning, Georgia, the unit's base of operation, of soldiers' deaths.After landing in the "Valley of Death", the soldiers capture a Vietnamese lookout who informs them that the location they were sent to is actually the headquarters of an entire North Vietnamese division. Another American squad is isolated at some distance from the battalion's main position, after 2nd Lt. Henry Herrick (Marc Blucas) saw a scout, and rashly ran after him, ordering his reluctant soldiers to follow. The scout leads them into an ambush, resulting in the majority of the platoon members' deaths, including Herrick's. Sgt. Savage (Ryan Hurst) assumes command of the squad, and by calling in artillery and using the cover of darkness, holds off the North Vietnamese from their position. The story switches between the Vietnamese and American points of view several times. Despite being trapped near the landing zone, and desperately outnumbered, the main force manages to hold off the enemy North Vietnamese with artillery, close air support, and even calling a last resort 'Broken Arrow' airstrike at their most desperate point, killing some of their own soldiers but eliminating most of the Vietnamese offensive force.The American troopers regroup, secure the area and charge up the mountain where the Vietnamese division headquarters is located. The Vietnamese have set up heavy gun emplacements near the hidden entrance of the underground headquarters spoken of by the scout. Hal and his men charge right at them, into a seemingly impending massacre, but before the Vietnamese can fire, Major Bruce "Snakeshit" Crandall (Greg Kinnear) flies in with his helicopter and kills the Vietnamese guards with his side mounted machine guns. Meanwhile, the Vietnamese commander is alerted that the Americans have broken through the lines, and the headquarters has no troops between them and the Americans. He orders the headquarters evacuated. The stranded platoon led by Savage are rescued. Moore, having completed his objective, returns to the L.Z. to be picked up, and, after all of his men, dead or alive, are removed from the battlefield, steps on to a helicopter and flies out of the valley. Strong visual emphasis is placed on Moore's being the last American to set foot off the field of battle. At the end of the movie it is revealed that Hal Moore returned home safely after 235 more days of fighting.
question: Who were trying to overrun their defensive position?
answer: The Vietminh soldiers.