The US was already providing help to the Brits. Declaring war on the US was dumb but from the view we were already involved essentially a formality on Germany’s part.
Still, got to stand in awe of the strategic incompetence of a guy who declares war on a nation his armed forces can’t even really attack, one with vast resources, but that will be able to attack him sooner or later.
Had he not done so it is possible that WW2 would have developed into two concurrent wars, at least until some later ”provocation” of our alliance with Britain against Japan caused Hitler to be dumb. So ... maybe a few more months ... tops.
Now Italy getting into the war ... THAT was over the top dumb as well as pointless. Old BM (strategically incompetent as they come, maybe even worse that Der Wanker) was hot to trot though to start some more regional wars he had no chance of winning. He also apparently thought he’d get left out of the payola from a presumably quick German victory.
As for Italian preparedness, by some accounts I’ve encountered the Italians had issues fueling their otherwise impressive fleet during peacetime!
Individual Italians are among the best people I know but the way they organize things leaves you speechless most of the time. One might wonder how they are capable to run their country at all, let alone make war.
If you are to look at Costa Concordia sinking that was an epitome of all things Italian from start to finish.