United States Army
20th century: Starting in 1910, the army began acquiring fixed-wing aircraft. In 1910, during the Mexican Revolution, the army was deployed to U.S. towns near the border to ensure the safety of lives and property. In 1916, Pancho Villa, a major rebel leader, attacked Columbus, New Mexico, prompting a U.S. intervention in Mexico until 7 February 1917. They fought the rebels and the Mexican federal troops until 1918.

World Wars: The United States joined World War I as an "Associated Power" in 1917 on the side of Britain, France, Russia, Italy and the other Allies. U.S. troops were sent to the Western Front and were involved in the last offensives that ended the war. With the armistice in November 1918, the army once again decreased its forces.
In 1939, estimates of the Army's strength ranged between 174,000 and 200,000 soldiers, smaller than that of Portugal's, which ranked it 17th or 19th in the world in size. General George C. Marshall became Army chief of staff in September 1939 and set about expanding and modernizing the Army in preparation for war.

The United States joined World War II in December 1941 after the Japanese attack on Pearl Harbor. Some 11 million Americans were to serve in various Army operations. On the European front, U.S. Army troops formed a significant portion of the forces that landed in French North Africa and took Tunisia and then moved on to Sicily and later fought in Italy. In the June 1944 landings in northern France and in the subsequent liberation of Europe and defeat of Nazi Germany, millions of U.S. Army troops played a central role. In 1947, the number of soldiers in the US Army had decreased from eight million in 1945 to 684,000 soldiers and the total number of active divisions had dropped from 89 to 12. The leaders of the Army saw this demobilization as a success.
In the Pacific War, U.S. Army soldiers participated alongside the United States Marine Corps in capturing the Pacific Islands from Japanese control. Following the Axis surrenders in May (Germany) and August (Japan) of 1945, army troops were deployed to Japan and Germany to occupy the two defeated nations. Two years after World War II, the Army Air Forces separated from the army to become the United States Air Force in September 1947. In 1948, the army was desegregated by order 9981 of President Harry S. Truman.

Cold War: 

1945–1960: The end of World War II set the stage for the East–West confrontation known as the Cold War. With the outbreak of the Korean War, concerns over the defense of Western Europe rose. Two corps, V and VII, were reactivated under Seventh United States Army in 1950 and U.S. strength in Europe rose from one division to four. Hundreds of thousands of U.S. troops remained stationed in West Germany, with others in Belgium, the Netherlands and the United Kingdom, until the 1990s in anticipation of a possible Soviet attack.: minute 9:00–10:00 

During the Cold War, U.S. troops and their allies fought communist forces in Korea and Vietnam. The Korean War began in June 1950, when the Soviets walked out of a UN Security Council meeting, removing their possible veto. Under a United Nations umbrella, hundreds of thousands of U.S. troops fought to prevent the takeover of South Korea by North Korea and later to invade the northern nation. After repeated advances and retreats by both sides and the Chinese People's Volunteer Army's entry into the war, the Korean Armistice Agreement returned the peninsula to the status quo in July 1953.

1960–1970: The Vietnam War is often regarded as a low point for the U.S. Army due to the use of drafted personnel, the unpopularity of the war with the U.S. public and frustrating restrictions placed on the military by U.S. political leaders. While U.S. forces had been stationed in South Vietnam since 1959, in intelligence and advising/training roles, they were not deployed in large numbers until 1965, after the Gulf of Tonkin Incident. U.S. forces effectively established and maintained control of the "traditional" battlefield, but they struggled to counter the guerrilla hit and run tactics of the communist Viet Cong and the People's Army Of Vietnam (NVA).

During the 1960s, the Department of Defense continued to scrutinize the reserve forces and to question the number of divisions and brigades as well as the redundancy of maintaining two reserve components, the Army National Guard and the Army Reserve. In 1967, Secretary of Defense Robert McNamara decided that 15 combat divisions in the Army National Guard were unnecessary and cut the number to eight divisions (one mechanized infantry, two armored, and five infantry), but increased the number of brigades from seven to 18 (one airborne, one armored, two mechanized infantry and 14 infantry). The loss of the divisions did not sit well with the states. Their objections included the inadequate maneuver element mix for those that remained and the end to the practice of rotating divisional commands among the states that supported them. Under the proposal, the remaining division commanders were to reside in the state of the division base. However, no reduction in total Army National Guard strength was to take place, which convinced the governors to accept the plan. The states reorganized their forces accordingly between 1 December 1967 and 1 May 1968.

1970–1990: The Total Force Policy was adopted by Chief of Staff of the Army General Creighton Abrams in the aftermath of the Vietnam War and involved treating the three components of the army – the Regular Army, the Army National Guard and the Army Reserve as a single force. General Abrams' intertwining of the three components of the army effectively made extended operations impossible without the involvement of both the Army National Guard and Army Reserve in a predominantly combat support role.  The army converted to an all-volunteer force with greater emphasis on training to specific performance standards driven by the reforms of General William E. DePuy, the first commander of United States Army Training and Doctrine Command. Following the Camp David Accords that was signed by Egypt, Israel that was brokered by president Jimmy Carter in 1978, as part of the agreement, both the United States and Egypt agreed that there would be a joint military training led by both countries that would usually take place every 2 years, that exercise is known as Exercise Bright Star.
The 1980s was mostly a decade of reorganization. The Goldwater-Nichols Act of 1986 created unified combatant commands bringing the army together with the other four military services under unified, geographically organized command structures. The army also played a role in the invasions of Grenada in 1983 (Operation Urgent Fury) and Panama in 1989 (Operation Just Cause).
By 1989 Germany was nearing reunification and the Cold War was coming to a close. Army leadership reacted by starting to plan for a reduction in strength. By November 1989 Pentagon briefers were laying out plans to reduce army end strength by 23%, from 750,000 to 580,000. A number of incentives such as early retirement were used.

1990s: In 1990, Iraq invaded its smaller neighbor, Kuwait, and U.S. land forces quickly deployed to assure the protection of Saudi Arabia. In January 1991 Operation Desert Storm commenced, a U.S.-led coalition which deployed over 500,000 troops, the bulk of them from U.S. Army formations, to drive out Iraqi forces. The campaign ended in total victory, as Western coalition forces routed the Iraqi Army. Some of the largest tank battles in history were fought during the Gulf war. The Battle of Medina Ridge, Battle of Norfolk and the Battle of 73 Easting were tank battles of historical significance.

After Operation Desert Storm, the army did not see major combat operations for the remainder of the 1990s but did participate in a number of peacekeeping activities. In 1990 the Department of Defense issued guidance for "rebalancing" after a review of the Total Force Policy, but in 2004, USAF Air War College scholars concluded the guidance would reverse the Total Force Policy which is an "essential ingredient to the successful application of military force".

21st century: On 11 September 2001, 53 Army civilians (47 employees and six contractors) and 22 soldiers were among the 125 victims killed in the Pentagon in a terrorist attack when American Airlines Flight 77 commandeered by five Al-Qaeda hijackers slammed into the western side of the building, as part of the September 11 attacks. In response to the 11 September attacks and as part of the Global War on Terror, U.S. and NATO forces invaded Afghanistan in October 2001, displacing the Taliban government. The U.S. Army also led the combined U.S. and allied invasion of Iraq in 2003; it served as the primary source for ground forces with its ability to sustain short and long-term deployment operations. In the following years, the mission changed from conflict between regular militaries to counterinsurgency, resulting in the deaths of more than 4,000 U.S. service members (as of March 2008) and injuries to thousands more.