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Answer by Mike DeAngelo, computer programmer and amateur military strategist:
You specifically did not include the other American states as part of the belligerent powers. I'll answer on the basis of an Europe, the Middle East, and Africa and Asia-Pacific alliance versus the U.S. I'll also assume that no side chooses to introduce WMDs into the equation. In that scenario, I believe the United States can win or at least force an indefinite stalemate.
The simple fact is that the United States, via its Navy and Air Force, has the ability to project power across the ocean to the shores of its enemies, but those enemies do not have a significant ability to project power beyond their own coastal waters.
There are only a few powers in the EMEA/AsiaPac alliance that can project a significant amount of power through their navies. The U.K. still has a fair amount of experience operating in “blue water,” and Russia has some nuclear submarines. The U.K.'s surface fleet would be neutralized in the first days of the war. The sub battle would last longer, but the U.S. would be able to bring its own superior sub force plus surface ships and aircraft to bear in the battle. Very early in the war, the U.S. would completely control the oceans and the airspace over the oceans.
Once control of the oceans is established, it is nearly impossible to wrest control of that away. The alliance would need to build ships capable of fighting on the ocean, but those ships need to be built in ports and shipyards that can't be hidden, and they take years to build. The U.S. can use strategic bombing aircraft and carrier battle groups to hit those resources. Building those resources while under attack is near impossible.
At the same time, the U.S. manufacturing base, shipyards, etc. are not similarly vulnerable. The EMEA/AsiaPac powers don't have many weapons that can hit those targets. The U.K., France, China, and Russia have ICBMs used for their nuclear forces, but they would be reluctant to convert those to conventional use since an incoming ICBM might be seen as a nuclear first strike and trigger a nuclear response. (The U.S. in recent years has studied using ICBMs as conventional weapons and come to the same conclusion.) The U.S. can continue manufacturing more ships and aircraft openly and without disruption.
As long as the U.S. maintains control of the oceans and airspace above the oceans, the U.S. itself is invulnerable, and U.S. forces essentially control when and where the battles take place. The EMEA/AsiaPac forces have little to no chance of breaking that stranglehold. Carrier battle groups can operate beyond the range of shore-based anti-ship missiles. If the EMEA/AsiaPac alliance does attempt to send ships into the ocean, it'll quickly be tracked and sunk by aircraft or U.S. submarines.
Even if U.S. forward bases are successfully attacked at the beginning of the war, many key islands will be isolated and taken by the U.S. for further use in controlling the oceans and preventing the EMEA/AsiaPac forces from building a navy. For example, EMEA/AsiaPac's land-based forces might be able to control the Mediterranean or the the Persian Gulf well enough to prevent the U.S. Navy from operating there. They might try to build up a navy in those waters. The U.S. will simply capture places like the Azores, Canaries, and Diego Garcia. Any navy built in those waters would have to fight its way out, then fight its way across thousands and thousands of kilometers of ocean before they could hope to attack the U.S. It is an impossible task.
The only way EMEA/AsiaPac could defeat the U.S. in a conventional war would be to field a navy and air force at the start of the war large enough and capable enough to challenge the U.S. Navy and U.S. Air Force for control of the oceans. That means it would need to develop a significant new capability before the start of the war.
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