This is probably an opposition party (Republican) criticism of the current U.S. president (Barack Obama) and his supporters (the Democratic party). Ours is still an American world, but it is presided over by a president who doesn't believe in American power. > The claim is that Obama either doesn't believe that the U.S. has power or, if it does, he doesn't believe that power should be used. The best lack all conviction while the worst are filled with passionate intensity—and a sense that the moment is theirs to seize. > The criticism here is that "the best" (i.e. the Republicans) lack the conviction of their beliefs to act on them and wield political power while "the worst" (i.e. the Democrats) are filled with passionate intensity (i.e. they hold their misguided and erroneous beliefs with passion and intensity) and "a sense that theirs is the moment to seize" (i.e. that, if they will only act, this is their moment in history). I will note that the second sentence could be interpreted the other way as well. Consider ... "the best" (i.e. the Democrats) lack the conviction of their beliefs to act on them and wield political power while "the worst" (i.e. the Republicans) are filled with passionate intensity (i.e. they hold their misguided and erroneous beliefs with passion and intensity) and "a sense that theirs is the moment to seize" (i.e. that, if they will only act, this is their moment in history). However, the first sentence gives you the clue that the paragraph intends to attack Obama and the Democrats and so the second sentence must be interpreted to be attacking the Democrats in order to maintain logical consistency with the first sentence.

good