Its principal formations were the 8th Army, under General Bernard Montgomery and 1st Army under Lieutenant General Kenneth Anderson. 8th Army had three British Army corps under its command which contained a variety of forces from the British Empire. They were British X Corps, British XIII Corps and British XXX Corps. They had fought across virtually the whole North African shore to the east of Tunisia after winning a victory at the Second Battle of El Alamein in November 1942. 1st Army also had three Corps, but the corps themselves were far more varied in national origin. Only one, British V Corps was of British origin. The other corps were US II Corps and French XIX Corps. 1st Army controlled the forces that had landed in Morocco and Algeria in November 1942 in the first of the great Allied amphibious assualts of the war, Operation Torch.
18th Army Group was faced by two German Armies, Panzerarmee Afrika under General Erwin Rommel and 5th Panzer Army under General Jurgen von Arnim. The two German commanders disliked each other, and so often strategy was not coordinated.
Both 1st Army and 8th Army had enjoyed very quick initial success in their campaigns after November 1942. Once they reached Tunisia two things halted them. One was overextension of lines of communication and the other was the greater concentration of German troops that the smaller defended area produced. 1st Army in particular received stinging blows from Rommel at the Kasserine Pass. Rommel's veteran formations slammed into II Corps and the green American troops did not perform well. It was only after the replacement of their commander by Major General George Patton and the Axis attack somewhat running out of steam due to supply difficulties that the situation was stabilised.
Following the Kasserine Pass engagement and an Allied consolidation, a fresh attack was launched. 1st Army lead the main attack, with 8th Army providing support along the eastern coast of Tunisia. That attack lead eventually to the surrender of Axis forces in Africa. 250,000 men were taken prisoner, a number equal to that at Stalingrad.
Alexander sent the message, "We are masters of the North African shore."
Shortly after the surrender, 18th Army Group was disbanded.