By January 1966, a CIA report had concluded that Bình Định Province on South Vietnam's central coast was 'just about lost' to communist forces. The assessment was not hyperbole. The People's Army of Vietnam's 3rd Division — two regiments of North Vietnamese regulars and one regiment of main-force Viet Cong guerrillas — controlled much of the province's land and much of its population of roughly 800,000. The Bồng Sơn plain, the coastal lowlands where most people lived and farmed, was largely under North Vietnamese influence. The South Vietnamese government's authority existed primarily on paper and in provincial capitals. Into this situation the 1st Cavalry Division (Airmobile) was ordered to conduct a campaign of hammer-and-anvil airmobile assaults — helicopters inserting troops to flush the North Vietnamese toward blocking positions — intended to break the PAVN 3rd Division's grip on the province.

General Harry Kinnard assigned the mission to Colonel Hal Moore's 3rd Brigade, the same commander who had led the 1st Cavalry at the Battle of Ia Drang three months earlier. The plan had two phases. First, a brigade-size task force would establish a forward base at Phu Cat on Highway 1 and patrol ostentatiously, as if the real target were elsewhere. Then the division would shift north to Bồng Sơn itself and launch a series of airmobile operations across the plain and into the An Lão Valley, where Kinnard believed the PAVN 3rd Division's headquarters was located. On January 28, Operation Double Eagle — the largest amphibious assault of the Vietnam War since Korea — sent U.S. Marines ashore to push south from Quảng Ngãi province, theoretically trapping PAVN forces between two advancing forces. The Marines found few enemy soldiers. The main force regiments had withdrawn days before the landing.

Operation Masher lasted from January 24 to March 6, 1966. Its commanders declared it a success, citing significant losses inflicted on the PAVN 3rd Division. But intelligence reports a week after the 1st Cavalry withdrew showed North Vietnamese soldiers already returning to the areas they had vacated. The discrepancy between weapons recovered and bodies counted raised questions about how many of the dead were actually combatants. An American journalist who visited a refugee camp housing 6,000 people displaced by the operation found them packed thirty to a room, receiving inadequate food and medical treatment, 'in a sullen and depressed mood.' Under the rules of engagement then in effect, anyone in a free-fire zone who did not leave voluntarily could be treated as Viet Cong. An unknown number of civilians died. During the subsequent Fulbright Hearings in the U.S. Senate, testimony alleged that for every PAVN or Viet Cong casualty reported, six civilian casualties had also occurred. The allegation helped accelerate growing public opposition to the war at home.

Bình Định Province would be fought over repeatedly in the years that followed. The same terrain, the same villages, the same coastal plain between Highway 1 and the South China Sea — Operation Washington Green would spend nearly two years there from 1969 to 1971, the Battle of Tam Quan would erupt in December 1967, and the Bồng Sơn plain would remain contested ground until the war's end. Operation Masher/White Wing demonstrated the central tension of the American strategy in Vietnam: the 1st Cavalry could clear any area it chose to enter, but it could not hold what it cleared. Clearing required firepower. Holding required legitimacy. The operation's name change, ordered because the original sounded too harsh, captured something true about the war's management: those in charge understood, on some level, that what was happening was difficult to defend. They changed the name. They did not change the operation.