A war with Russia would probably turn nuclear and that would in turn lead to "the end of humanity" as we know it. A no win situation.

A war with China would also turn nuclear, although we could probably survive that one. An invasion would be impossible due to their overwhelming population and nationalism, similar to Japanese in WW2. Their huge population would be extremely susceptible to food, water and material shortages. That would be the way to bring them to their knees. We could probably win this one, but it would be an extremely difficult challenge and could take many years to achieve.

We could probably kick Iran's ass quickly once we destroy their front line defenses. Destroy their manufacturing and transportation infrastructure, eliminate their water supply, then starve them to death. We could win this one but forget about post war reconciliation or rebuilding

But then again, any of the above could escalate into a Armageddon type scenerio, and then the whole world looses.

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