According to The Washington Post, he says "Obama is becoming the antiwar candidate, and Hillary Clinton is becoming the responsible Democrat who could become commander in chief in a post-9/11 world." One can try to speculate that Kristol is
playing some odd angles here, but I think the record indicates that he's genuinely more committed to war -- criticized Republican critics of the Kosovo War, criticized Bill Clinton for not killing enough people during the Kosovo War, backed John McCain in the 2000 primaries -- and based on the evidence thinks Clinton will be more sympathetic to his agenda than the alternatives.
This article available online at:
http://www.theatlantic.com/politics/archive/2007/08/clinton-wins-coveted-bill-kristol-endorsement/45706/