The United States Air Force got its start in the dual creation of the Air Service wing of the American Expeditionary Force and the Army Air Service, both in 1918, shortly after the United States had entered the First World War. During the war they mainly provided air support for US Army ground forces. When the war ended in November of 1918 the Air Service of the AEF was dissolved, and the Army Air Service, now under direct control of the United States Secretary of War, mostly demobilized. In 1920 the Air Service became a branch of the Army, and in 1926 was renamed as the Army Air Corps. The Air Corps became increasingly involved in technological innovations and the development of new aircraft during this period. In 1935 the various air forces, until then under the control of separate ground commands, was brought under the command of the General Headquarters Air Force.
When the United States entered the Second World War in 1941, the Air Force got a large part of the autonomy that they had been campaigning for since the mid-1920s. The Air Corps was brought under the command of the newly-created Army Air Forces, which by 1942 had been awarded equal standing with the Army and the US Navy, as well as freedom from the command of the ground forces. Into 1943 they began a strategic bombing campaign into Germany, however the strategy was imperfect, and unacceptable losses caused a scaling down of air campaigns by the autumn of that year. The Air Force began focusing on aircrafts that would allow for greater range and would be more able to fly at higher altitudes. By late February of 1944 the Air Force was able to take down large swathes of the Luftwaffe, and waged unrestricted bombing campaigns against German airfields. They were also involved in the Pacific Theatre, where they acted in mainly a support capacity for ground and naval troops, as well as inventing the skip bombing technique, in which planes would fly low and target Japanese ships, which proved immensely successful. Between 1942 and 1945 they were involved in airlifts in China and India. They also waged incendiary bombing campaigns against large Japanese cities, most notably Tokyo, and in 1945 dropped the atomic bombs on Hiroshima and Nagasaki, effectively ending Japanese resistance and the war.
The US Air Force officially became a separate branch of the US military on September 18th, 1947 under the National Security Act of 1947. During this time the US and Soviet Russia became embroiled in the Cold War, and the United States began to rapidly escalate the presence of its military around the world. To this end, Air Force bases were established in various countries throughout Europe, as well as South Korea, Japan, British Indian Ocean territory and Ascension Island. 1947 also saw the first official interest of the Air Force in unidentified flying objects, as Project Sign. In 1948 the Communist forces controlling East Germany cut West Berlin off from land and air support. The Air Force, in conjunction with the Royal Air Force, began the Berlin airlift, later known as Operation Vittles, to provide provisions to the city. This action not only saved the city from starvation, but forced the Soviets to back down from the blockade.
In 1950 the Korean War began, and the Air Force unit stationed there, the Far East Air Force (FEAF), was among the first to respond. They soon lost their base at Kimpo, South Korea, however, and were forced instead to provide close sir support, as well as conducting bombing runs against North Korea. In September of that year, the landing at Inchon provided them again with a base in Korea, from which they provided support for General Douglas MacArthur as he pushed towards the Korean-Chinese border, and again after the People's Liberation Army from China attacked. The Air Force's bombing and air support missions during this period were successful, though attempts to interrupt North Korean supply lines failed, and was eventually replaced with bombing campaigns intended to rack up as much economic damage against the Chinese and North Koreans as possible.
The Vietnam War began two years after the end of the Korean War, in 1955. The Air Force conducted their first bombing campaign there in 1964, and remained heavily involved in the war after that. In 1965 Operation Rolling Thunder was commenced, a sustained aerial bombardment campaign that was intended to quash North Vietnamese resistance and supply lines, and lasted for a little over three years. It destroyed a large part of the infrastructure in North Vietnam, and inflicted major economic loss, but was otherwise unsuccessful. In 1972 the Air Force was heavily involved in the Easter Offensive, and their swift mobilization was in large part what aided the South Vietnamese in pushing the invaders back. This was conducted along with Operation Linebacker, which aimed to cut off supplies to the North Vietnamese forces who were involved in the Easter Offensive. In late December of 1972 Operation Linebacker II, also known as the Christmas Bombings, was conducted against various North Vietnamese targets, notably Hanoi and Haiphong where over 1600 civilians died. This marked the Air Force's last operation in the Vietnam War.
In 1980 the Air Force was involved in the disastrous rescue attempt of Operation Eagle Claw, which was meant to rescue 52 American citizens trapped in Iran, and which ended with the deaths of eight American servicemen, five of whom were members of the Air Force, as well as the loss of a helicopter and a transport aircraft. Between 1983 and 1989 they were involved in Grenada, Libya and Panama, where they fought as well as providing airlifts and air support. During the Gulf War they provided air support, were involved in Operation Desert Storm, and after the fighting patrolled the skies along with the RAF and the Navy aircrafts to ensure that Iraq's defense capabilities remained destroyed. Between 1991 and 1996 they were involved in Operation Provide Comfort, in 1996 with Operation Desert Strike and Operation Desert Fox two years later, and in Operation Northern Comfort between 1997 and 2000, all in Iraq, either enforcing no-fly zones or as part of a bombing campaign. Also during this period they were involved in Bosnia, again providing no-fly support between 1993 and 1996 as part of Operation Deny Flight, and air strikes in 1995 and 1999 as Operation Deliberate Force and Operation Allied Force, respectively.
On September 11th, 2001, the United States was attacked by terrorists. The US Air Force, along with other elements of the US military and NATO, became involved in the Global War on Terror. They were involved in the 2001 invasion of Afghanistan and subsequent operations against the Taliban, and opened bases in Central Asia for the first time. Two years later, they were involved in the invasion of Iraq, taking over the Baghdad International Airport as a base after the defeat of Saddam Hussein's forces. They maintain a base in Qatar to direct air operation in the region. As of 2008 the Air Force has begun a significant expansion effort to replace aging equipment.
Currently, the US Air Force is headquartered at The Pentagon. It is divided into three major components: the active duty forces, the Air Force Reserve and the Air National Guard. Altogether it contains 302 fighting squadrons. Currently there are just under 330,000 personnel on active duty, 69,000 in the Reserve, 95,000 in the Air Guard, as well as just under 169,000 civilian personnel. As well it has 5,573 aircraft of which just over 2,100 are fighters, 450 ICBMs and 32 satellites.