A French unit on patrol in Vietnam in 1954, during the final year of the First Indochina War, is ambushed by Viet Minh forces, probably the Battle of Mang Yang Pass. Viet Minh commander Nguyen Huu An (Đơn Dương) orders his soldiers to “kill all they send, and they will stop coming”.
Eleven years later, the United States is fighting the Vietnam War. U.S. Army Lieutenant Colonel Hal Moore (Mel Gibson) is chosen to train and lead a battalion. After arriving in Vietnam, he learns that an American base has been attacked, and is ordered to take his 400 men after the enemy and eliminate the North Vietnamese attackers, despite the fact that intelligence has no idea of the number of enemy troops. Moore leads a newly created air cavalry unit into the Ia Drang Valley. After landing, the soldiers capture a North Vietnamese soldier and learn from him that the location they were sent to is actually the base camp for a veteran North Vietnamese army division of 4,000 men.
Upon arrival in the area with a platoon of soldiers, 2nd Lt. Henry Herrick (Marc Blucas) spots an enemy scout and runs after him, ordering his reluctant soldiers to follow. The scout lures them into an ambush, resulting in several men being killed, including Herrick and his subordinates. The surviving platoon members are surrounded and cut off from the rest of the battalion. Sgt. Savage (Ryan Hurst) assumes command, calls in artillery, and uses the cover of night to keep the Vietnamese from overrunning their defensive position.
Meanwhile, with helicopters constantly dropping off units, Moore manages to secure weak points before the North Vietnamese can take advantage of them. Despite being trapped and desperately outnumbered, the main U.S. force manages to hold off the North Vietnamese with artillery, mortars, and helicopter airlifts of supplies and reinforcements. Eventually, Nguyen Huu An, the commander of the North Vietnamese division, orders a large-scale attack on the American position.
At the point of being overrun by the enemy, Moore orders 1st Lt. Charlie Hastings (Robert Bagnell), his Forward Air Controller, to call in “Broken Arrow” (a call for all available combat aircraft to assist and attack enemy positions, even those close to the U.S. troops’ position, because a position is being overrun and can no longer be defended). The aircraft attack with bombs, napalm, and machine guns, killing many PAVN and Viet Cong troops; but a friendly fire incident also results in American deaths. The North Vietnamese attack is repelled, and the surviving soldiers of Herrick’s cut-off platoon, including Savage, are rescued.
Meanwhile, back in the United States, Julia Moore (Madeleine Stowe) has become the leader of the American wives living on the base. When the Army begins to use yellow cab drivers to deliver telegrams notifying the next of kin of soldiers’ deaths in combat, Julia personally assumes that emotional responsibility instead.
Moore’s troops regroup and secure the area. Nguyen Huu An plans a final assault on the Americans and sends most of his troops to carry out the attack, but Moore and his men overrun them and approach the enemy command center. Before the base camp guards can open fire, Major Bruce “Snake” Crandall (Greg Kinnear) and others helicopter gunships attack, destroying the remnant of the enemy force. With no more troops to call on, Huu An quickly orders the headquarters evacuated.
Having achieved his objective, Moore returns to the helicopter landing zone to be picked up. Only after everyone (including the dead and wounded) is removed from the battlefield does he fly out of the valley. Some time later, Nguyen Huu An and his men arrive on the battlefield to collect their dead. He claims that the Americans will “think this was their victory. So this will become an American war.”
At the end of the film, it is revealed that the landing zone immediately reverted to North Vietnamese hands after the American troops were airlifted out. Hal Moore continued the battle in a different landing zone, and after nearly a year he returns home safely to Julia and his family. His superiors congratulate him for killing over 1,800 North Vietnamese Army and Viet Cong soldiers. An older Moore visits the Vietnam War memorial and looks at the names of the soldiers who fell at Ia Drang.