This is one hundred percent hypothetical, so bear with me[url=http://lh6.ggpht.com/__zoKJ77EvEc/TJJU_wxYXPI/AAAAAAAAHhM/Fdy3Kc9RNWs/bear-yoga%20(1)%5B2%5D.jpg?imgmax=800].[/url]
Let's pretend that for whatever reason, each country simultaneously declares war on each other the second you read this. Each country's eventual goal is to eliminate the other, or to absorb the other country into their own empire.
[b]What would happen first? Who would each country ally with? Who would win in the end?[/b]
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No one wins. Both have enough nukes to wipe out all life on Earth. If they were fully committed to the destruction of the other, the nukes would fly and we'd all die.