One of the standard tropes of American politics is about global leadership. The automatic assumption on nearly all pressing issues is that we lead, and one of the charges typical of presidential campaigns is that the other candidate doesn't, can't or won't lead. But now, on the most momentous concern of our time, a sizable number of the political class -- including White House aspirants -- are insisting we forego global leadership because the United States doesn't matter to the fate of the issue.