if literally every single other country in the entire world decided to invade the USA together, will all of their resources, would the US be able to hold off the attack or would it fall?

Would fail. The level of coordination needed could not be achieved. Military might is useless with no viable way to wield it cohesively. This is assuming no nuclear weapons are used by either side.

At most, Alaska and Hawaii could be taken and held by a determined foe. But the mainland? No.