From north to south in Europe there were the British 1st Army and Canadian 1st Army (these made up the 21st Army Group, under Montgomery); then the US 9th, 1st and 3rd Armies
…(these were the 12th Army Group, under US General Omar Bradley); then there were the American 7th Army and French 1st Army (these were the 6th Army Group, under US General Jacob Devers). All three of these Army Groups were under Eisenhower's command. The German attack came at the boundary between the US 9th and 1st Armies, falling more on the 1st Army. These armies became separated when the Germans broke through. Because it looked as though the Germans might reach a small town which was a vital telephone line hub, which would make it difficult for Bradley to communicate with his 9th Army Commander, and for this reason ONLY, during the Battle the US 9th Army was placed under the direction of Montgomery, who was himself north of the Bulge. But the 9th Army was not really in the Ardennes Forest proper. They reformed and faced southward along Elsenborn Ridge. to make a northern shoulder of the Bulge, to contain the Germans and canalize them into moving forward only on the width of their initial breakthrough. Most of the troops actually in the Ardennes Forest were of the US 1st Army, which was commanded by General Courtney Hodges, and which remained in Omar Bradley's 12th Army Group, all under Eisenhower. (MORE) 
