The US Third Army was first activated as a formation during the First World War. However, since it was formed only four days before the armistice of 11 November 1918, it saw no fighting. Instead of wartime operations, occupation duties called for the men of the Third Army. They moved into central Germany, and began to enforce the terms of the armistice. A crisis ensued when there was a question as to whether the German Government would sign the Treaty of Versailles . In May 1919, General Ferdinand Foch went as far as to circulate plans for offensive operations to begin again against Germany. However this was not needed. Third Army was disbanded in July 1919, as the need for a headquarters of that level to control the American occupation forces in Germany was not evident.

Third Army did not see the light of day again until 1932. On 9 August of that year, in a reorganisation of field forces in the United States, four field armies, Third Army amongst them, were activated, to control the formations of the US Army stationed on home soil. Until the buildup of American forces prior to its entry into World War II, Third Army remained largely a paper formation. It held training exercises periodically, but these were almost never adequate.

Mobilisation saw Third Army take on the role of training some of the huge numbers of recruits that the draft was bringing into the Armed Forces. Lieutenant General Walter Krueger, later to gain fame for his command of Sixth Army during operations in the Pacific commanded Third Army from May 1941 until February 1943. Under his leadership, the basis of the Army's later success as a combat formation was laid. Krueger was succeeded by Lieutenant General Courtney Hodges who led the Army for the rest of 1943. The news that many had expected came in December 1943. Third Army was shipped from the US to the United Kingdom.

Third Army did not take part in the initial stages of Operation Overlord. However, when it did take the field, its field of combat suited the style of its commander far more. Lieutenant General George Patton was one of the US Army's greatest exponents of armoured warfare. When Third Army was moved to France, it was just after Bradley's formations had achieved the breakout from Normandy. Third Army followed up on that success and began a great dash across France. It was only the inevitability of logistics problems that halted Patton's force near the borders of Germany.

After a period of consolidation, Third Army was ready to on the offensive again. However, the Germans then launched their last great attack of WWII. The Battle of the Bulge saw an attempt to repeat the decisive breakthrough of 1940. However, in 1944, the Germans were doomed to failure. Their own logistical problems surfaced, and they ground to a halt. Nevertheless, they had broken the US front, and it took a great effort to reduce the resulting salient. In one of the great moves of the war, Patton turned Third Army's axis of advance through ninety degrees and set it upon the south of the German forces. The German salient was reduced by the end of January 1945, and the remainder of the process of closing up to the Rhine could be completed. Some vicious fighting took place, but by April there was but one great natural barrier between Third Army and the heart of Germany. Unlike in 1918, the crossing of the Rhine was opposed. However, the bridgehead was won, and Third Army embarked on another great eastward dash. It reached Austria and in May liberated the Mauthausen-Gusen concentration camps complex. Its forces ended up in Czechoslovakia, the furthest east of any American units.

Occupation beckoned again, and Third Army took up the challenge of starting to rebuild postwar Germany. Third Army remained in Germany until recalled to the United States again 1947. When back in the US, its duties were much the same as those of the 1930s, acting as a command and training force for units in the United States. The Korean War saw a repeat of the earlier WWII training duties. Third Army remained responsible for this aspect of US Armed Forces operations until 1974, when a new major headquarters, that of Forces Command, or FORSCOM was activated to replace Third Army. Third Army was thus inactivated, and remained so for the best part of a decade.

1982 saw the rebirth of Third Army as the ground forces component of the newly formed US Central Command (CENTCOM). It was not until 1990 that Third Army returned to combat, but it was a very notable return. Saddam Hussein invaded Kuwait in August 1990, and American forces were immediately despatched to Saudi Arabia to protect that kingdom. Since Saudi Arabia came within the CENTCOM area, Third Army was sent to command the Army units in theatre. At first, XVIII Corps made up the forces assigned to Third Army; only enough men to ensure that the Iraqis could not invade Saudi Arabia. However, in November 1990, massive reinforcements were announced in the form of VII Corps from Germany. This deployment marked the largest use of armoured formations by the US since WWII, and thus it was fitting that Patton's old command, Third Army, should have control of the battle. By the opening of hostilities, XVIII Corps had three American and one French division and VII Corps four American and one British divisions under command, thus giving Third Army a total of nine divisions under its command, plus the armored cavalry regiments attached to both corps.

Third Army was the main striking force in Operation Desert Storm. Its units were on the left flank of the attacking force and swept into southern Iraq. They then turned east and engaged the Iraqi Republican Guard in fierce combat. Much of that force was destroyed. In terms of its immediate aims, The Gulf War was a stunning success. The Iraqis were ejected from Kuwait and their forces were thoroughly mauled. However, over the longer term, it became clear that more operations would eventually be necessary. Throughout the 1990s, there was indecision on the part of the American Government over whether those operations should take place or not.

All that changed in September 2001. The terrorist attack on the World Trade Center provoked a fresh look at American national security. The immediate consequence of this attack was the invasion of Afghanistan in late 2001, which was too small an operation to require the deployment of Third Army. However, following the ousting of the Taliban, US attention returned to Iraq. Saddam Hussein had been a thorn in the US side for over a decade, and the reassessment of the threat he posed after 9/11 meant that it was now seen as essential to get rid of him.

After several months of diplomatic maneouvre, Third Army was deployed in early 2003. The forces it had under its command for Operation Iraqi Freedom were much smaller in numbers than those it had commanded twelve years before. It had V Corps as its main striking force, with only two complete divisions and an airborne brigade under that command. There was also I MEF , controlling a further two divisions and a brigade. However, numbers were made up for by the advances in technology which rendered this force one of incredible power. It took six weeks to complete the conquest of Iraq, with 3rd Infantry Division, the heavy armour component of V Corps moving faster than even Patton had managed during his great dash across France.

The aftermath of the campaign saw Third Army headquartered in Baghdad, directing its third occupation within one hundred years.