We Were Soldiers

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 orders his soldiers to "kill all they send, and they will stop coming".
Eleven years later, the U.S. 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. As the haunting lament "Sgt. MacKenzie" plays, Moore leads a newly created air cavalry unit into the Ia Drang Valley. After landing in the "Valley of Death", the soldiers learn 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 spots an enemy scout, runs after him, and orders reluctant soldiers to follow. The North Vietnamese scout lures them into an ambush, resulting in several men being killed, including Lt. Herrick and his subordinates. The surviving platoon members are surrounded with no chance of retreat. Sgt. Savage assumes command, calls in artillery, and uses the cover of night to keep the Vietnamese from overrunning their small defensive position. Meanwhile, with helicopters constantly dropping off units, Lt. Col. Moore manages to secure weak points before the North Vietnamese can take advantage of them.
On the second day, 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, North Vietnamese commander Nguyen Huu An orders a large-scale attack on the American position.
At the point of being overrun by the enemy and with no options left, Moore orders 1st Lt. Charlie Hastings, 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 results in American deaths. The second North Vietnamese attack is repelled, and the surviving U.S. soldiers, led by Sgt. Savage, are rescued.
Meanwhile, back in the U.S., 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, secure the area, and stop at the base of a hill. The North Vietnamese commander plans a final assault on the Americans and sends most of his troops to carry out the attack. The North Vietnamese have set up strong emplacements near the hidden entrance of the underground passage to the command post, spoken of by the scout. Hal and his men charge at them, but before the North Vietnamese can fire, Major Bruce "Snake" Crandall and others in helicopter gunships attack the North Vietnamese, destroying the bulk of the enemy force.
Nguyen Huu An, the North Vietnamese Commander, is alerted that the Americans have broken through their lines, and there are no soldiers between the Americans and their command post. Since the Commander had deployed his reserve forces to a final offensive, and the base camp has no troops to call upon for defense, the North Vietnamese commander quickly orders the headquarters evacuated.
Moore, having achieved his objective, 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.
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 sees the names of soldiers who fell at Ia Drang.

Question: What battle was this?
Answer: the Battle of Mang Yang Pass