A Fascist-ish (do not expect a carbon copy of Italy, Germany, or Japan. Rather, expect a nationalistic, racist, nativist, pro-business conglomerate.) U.S would be mostly concerned with internal affairs rather than expansion. At the very least we'd see promotion of the KKK and other groups. At worst we could see the expansion of eugenics to genocidal levels.
You have to look at the American far right of this time to get an idea. They would be wildly nativist and concerned with keeping civil rights down. For any regime to be successful, however, they would have to cooperate at least somewhat with business of the time, making them unconcerned with smashing everything about the old order.
As for foreign affairs, don't expect massive expansionist campaigns through North America- nobody's interested. In fact, a fascist America's main event would probably be Japan, and they would destroy them to solidify America's control over the Pacific. Meanwhile, Latin America would slowly adopt governments acceptable to a far right America.
The Soviet Union would remain the big Boogeyman America despises. Assuming Europe goes the same as OTL, they'll try to pressure the Nazis and Britain into making peace, while supporting the Germans against the USSR (similar to how they helped the entente in ww1).
Overall, they'll be concerned primarily with reshaping U.S society into what they want it to be. What this is depends on specifics. Meanwhile, they will strengthen their hold over U.S spheres of influence, including Latin America, the Pacific, and Southeast Asia (including China). They will mostly ignore Europe, except to contain the SU and possibly become friends with Nazi Germany.
It's hard to tell whether the far right's love affair with Nazi Germany would persist ITTL. It probably would, but a fascist U.S would have very little problems with Britain.
