He is not the commander. The German is USAREUR Chief of Staff. I think it is stupid, but he is not a commander.
During the Bulge, Bradley’s First and Ninth Armies were placed under command of Montgomery. Eisenhower claimed it was for tactical and logistical reasons, but many suspect, Eisenhower saw more fight in Montgomery. Montgomery, who was charged with keeping pressure on the German northern flank, performed well enough, but afterwards, when he seemed to take credit for the victory in the Bulge, he infuriated American commanders, to the point where Eisenhower demanded that Montgomery or he resign. Churchill intervened, and Ike settled for a public apology from Montgomery. (I suspect if Patton had been in command in the Ardenne prior to the German assault, they never would have enjoyed even the limited successes that they did.)