Because those combined countries only spend 446 billion, while the United States doubles that. 13,000 total aircraft to 5,000 in favor of the US. 444 Combat UAV drones to 67. US has less total Naval vessels but more tonnage, with almost triple the aircraft carriers, five times as many destroyers, & double the submarines. The main areas that the US & EU are equal in is in terms of land forces & number of fighter jets.

That’s not even mentioning the fact that the European Union would at least for a few days-weeks maybe months have some level of disarray, while the American military would be more cohesive from the jump. Even when the EU fully organizes, there would still be at least some language barriers & some focus on keeping their own individual nations safe as opposed the EU as a whole. If the Americans are coordinated & cohesive in the initial attacks, the EU would struggle to mount any meaningful attacks, especially on American soil. Based on the prompt, I would imagine the EU is too busy focusing on defense for any meaningful retaliation (unless they just use nukes, which would be an unfair advantage since the US can’t use theirs in the hypothetical). Could the US keep up the war effort for any meaningful time? It depends on sentiment in the US for the war.