This is a very good question. American soldiers who encountered Germans during the World War II, have found the task extremely hard. It was no easy victory.

One of the greatest issues was that German soldiers were very good fighters. They had been fighting several years earlier and even junior leaders, sergeants and lower, could improvise on the spot without orders being given. This made them a very formidable enemy, particularly to the untrained American soldiers.

The tanks of the Americans also lacked enough power. The main tank the Sherman was good but it was unable to pierce the heavy German tanks like the tanks Tiger and Panther. German guns would easily pierce through Sherman armor, which crews on the American side were terrified.

Environment and climate were also a serious challenge. The winter years of 1944 were very cold and wet and most American GIs contracted trench foot. Their feet got inflamed and infected. Air power and big artillery were of no assistance in thick vegetation like the Hurtgen Forest resulting in tough and bloody infantry fighting.

Finally, the US won because of an extensive supply system and absolute air dominance. These elements slowly suffocated the strong German army with time.