United States of America kept unaware of the WWII until the attack of Pearl Harbor, carried out by Japan forces. After that, United States fight back the Axis power, first in Pacific against Japan, and later in North Africa, Italy, and the rest of Europe.
The US focus on offence, as most units are equipped to deal damage fast enough before something is able to kill them. Many, many tactics can be employed by the US, as they have a lot of options by midgame and switching tech doesn't seem to throw US back all that much.
While offence is good, there are times when you need to protect yourself from invading forces. Unfortunatly, US don't really defend all that well. Often a defending US player will retreat to get more breathing room, while other factions can hold any sort of ground comfortably. Shorter ranged units have this behaviour in general.
While US have a solid midgame and a very aggresive start with powerful infantry and much better Shermans, they lose a lot of that momentum once other factions get to catch up in the tech tree, making Shermans not too good in general.