Unless things change, the man (or woman) elected in 2012 will be the last American president to preside over the world's leading economy. If things get really bad, he will find himself presiding over the early stages of American collapse. Not "decline" but "collapse." "Decline" is what happens when you're Britain in the 1940s and you cede global dominance to a major ally that shares your language, legal system, cultural inheritance and broad geopolitical objectives. That deal isn't on offer this time round. Nor was the United Kingdom circa 1948 in thrall to anything like the same levels of spendaholic...