The Fairchild Republic A-10 Thunderbolt II, also widely known by the nickname A-10 Warthog, is a single-seat, twin-turbofan, straight-wing, subsonic attack aircraft developed by Fairchild Republic for the United States Air Force (USAF). In service since 1977, it is named after the Republic P-47 Thunderbolt strike-fighter of World War II, but is instead commonly referred to as the "Warthog" (sometimes simply "Hog"). The A-10 was designed to provide close air support (CAS) to ground troops by attacking enemy armored vehicles, tanks, and other ground forces; it is the only production-built aircraft designed solely for CAS to have served with the U.S. Air Force. Its secondary mission is to direct other aircraft in attacks on ground targets, a role called forward air controller (FAC)-airborne; aircraft used primarily in this role are designated OA-10.

The A-10 was intended to improve on the performance and firepower of the Douglas A-1 Skyraider. The Thunderbolt II's airframe was designed around the high-power 30 mm GAU-8 Avenger rotary autocannon. The airframe was designed for durability, with measures such as 1,200 pounds (540 kg) of titanium armor to protect the cockpit and aircraft systems, enabling it to absorb damage and continue flying. Its ability to take off and land from relatively short and/or unpaved runways permits operation from airstrips close to the front lines, and its simple design enables maintenance with minimal facilities.

It served in the Gulf War (Operation Desert Storm), the American-led intervention against Iraq's invasion of Kuwait, where the aircraft distinguished itself. The A-10 also participated in other conflicts such as the Balkans, Afghanistan, the Iraq War, and against the Islamic State in the Middle East.

The A-10A single-seat variant was the only version produced, though one pre-production airframe was modified into the YA-10B twin-seat prototype to test an all-weather night-capable version. In 2005, a program was started to upgrade the remaining A-10A aircraft to the A-10C configuration, with modern avionics for use with precision weaponry. The U.S. Air Force had stated the Lockheed Martin F-35 Lightning II would replace the A-10 as it entered service, but this remains highly contentious within the USAF and in political circles. The USAF gained congressional permission to start retiring A-10s in 2023, but further retirements were paused until the USAF can demonstrate that the A-10's close-air-support capabilities can be replaced.

The development of conventionally armed attack aircraft in the United States stagnated after World War II, as design efforts for tactical aircraft focused on the delivery of nuclear weapons using high-speed designs such as the McDonnell F-101 Voodoo and Republic F-105 Thunderchief. As the U.S. military entered the Vietnam War, its main ground-attack aircraft was the Korean War-era Douglas A-1 Skyraider. A capable aircraft for its era, with a relatively large payload and long loiter time, the propeller-driven design had become relatively slow, vulnerable, particularly to ground fire, and incapable of providing adequate firepower. The U.S. Air Force and Navy lost some 266 A-1s in action in Vietnam, largely from small-arms fire.

The lack of modern conventional attack capability prompted calls for a specialized attack aircraft. On 7 June 1961, the Secretary of Defense Robert McNamara ordered the USAF to develop two tactical aircraft, one for the long-range strike and interdictor role, and the other focusing on the fighter-bomber mission. The former was the Tactical Fighter Experimental (TFX) intended to be a common design for the USAF and the US Navy, which emerged as the General Dynamics F-111 Aardvark, while the second was filled by a version of the U.S. Navy's McDonnell Douglas F-4 Phantom II. While the Phantom went on to be one of the most successful fighter designs of the 1960s and proved to be a capable fighter-bomber, its short loiter time was a major problem, as was its poor low-speed performance, albeit to a lesser extent. It was also expensive to buy and operate, with a flyaway cost of $2 million in FY1965 ($20 million today), and operational costs over $900 per hour ($9,000 per hour today).

After a broad review of its tactical force structure, the USAF decided to adopt a low-cost aircraft to supplement the F-4 and F-111. It first focused on the Northrop F-5, which had air-to-air capability. A 1965 cost-effectiveness study shifted the focus from the F-5 to the less expensive A-7D variant of the LTV A-7 Corsair II, and a contract was awarded. However, this aircraft doubled in cost with demands for an upgraded engine and new avionics.

During this period, the United States Army had been introducing the Bell UH-1 Iroquois into service. First used in its intended role as a transport, it was soon modified in the field to carry more machine guns in what became known as the helicopter gunship role. This proved effective against the lightly armed enemy, and new gun and rocket pods were added. Soon the Bell AH-1 Cobra was introduced. This was an attack helicopter armed with long-range BGM-71 TOW missiles able to destroy tanks from outside the range of defensive fire. The helicopter was effective and prompted the U.S. military to change its defensive strategy in Europe into blunting any Warsaw Pact advance with anti-tank helicopters instead of the tactical nuclear weapons that had been the basis for NATO's battle plans since the 1950s.