In WW2 they certainly did. This was not helped by the American defeat at Kasserine Pass in North Africa, in which large numbers of American soldiers broke and ran before the German advance. However, it is accurate to say that American leadership were themselves doubtful of their own capabilities. The British had been fighting the Germans for a year and a half by the time the US entered the war and were largely successful in dictating strategy until late 1943 when American confidence grew enough to insist on the Normandy invasion in 1944. Admittedly, soviet insistence on a second front in France played a role here, too. Upon joining the war, American planners wanted to immediately attach over the channel. However, the British convinced American leadership that they were not ready for such a committed attack and that they should instead invade North Africa almost as a training exercise. After success there, Churchill was successful in convincing Roosevelt that attacking 'the soft underbelly' through Italy was the best way to expedite the end if the war, even though many American generals (somewhat correctly) believed that attacking through Italy would have little strategic value and (more correctly) that it would be extremely difficult due to the mountainous terrain favoring the defenders and negating the armor advantage of the Allies. Italy was a complete slog, and while it knocked the Italians out of the war, it did not take as many German divisions as expected to slow the allied advance to a standstill until 1944 when the jig was already up.

Source: Rick Atkinson, Liberation Trilogy