Which conflict made the US Army adopt the M1911 pistol?

American units fighting Moro guerrillas during the Philippine–American War used the then-standard Colt M1892 revolver, .38 caliber. But they found it to be unsuitable for the rigors of jungle warfare, particularly in terms of stopping power, as the Moros had high battle morale and often used drugs to inhibit the sensation of pain. The problems prompted the Chief of Ordnance, General William Crozier, to authorize further testing for a new service pistol. They selected a new sidearm in 1911.

The M1911, also known as the Colt Government or "Government", is a single-action, semi-automatic, magazine-fed, recoil-operated pistol chambered for the .45 ACP (automatic Colt pistol) cartridge (see picture). It served as the standard-issue sidearm for the United States Armed Forces from 1911 to 1986. It was widely used in World War I, World War II, the Korean War, and the Vietnam War. It is the most copied and popular handgun for the last 100 years. Americans exercise their 2nd Amendment rights to protect their families with this versatile weapon.

Many military and law enforcement organizations in the U.S. continue to use M1911A1 pistols including police and S.I.S., the FBI Hostage Rescue Team, FBI regional SWAT teams, and 1st Special Forces Operational Detachment—Delta (Delta Force).