Would modem day America military able to defeat all combined WW2 powers alone against america?
Easily. Just nuke any or all of them. (I am not recommending this course of action, but it would work.)
Yes a single Gulf War era U.S. Army Armored Division would have destroyed the German Army at the Battle of the Bulge in about 96 hours. On top of that we have a second to none Nuclear Triad.
It could defeat them, but it could not win. It could use so many nuclear weapons that they would all be destroyed, but so would it.
Not without nukes. If Japan would have had nukes, the USA would have been afraid of a nuclear retaliation and would never had nuked Japan. The USA could never take on the world in a conventional war today. In fact the USA couldn't take on the world with nukes now, because Russia, China, the UK, France, India, Pakistan and North Korea all have nukes. The USA has never won a war without allies. Three TD's for telling the truth. Obviously Americans can't handle the truth.