SEATTLE, May 27 - Senator John Kerry mounted a broad new attack on the Bush administration's handling of national security on Thursday by accusing the president of undermining "the legacy of generations of American leadership" with a foreign policy that has abandoned the alliance-building of the post-World War II era. Opening a two-week critique of administration foreign policy, Mr. Kerry sought to present a clear alternative to Mr. Bush's approach to Iraq and the war on terrorism, while pursuing the same central goal: destroying Al Qaeda and its allies. "Let there be no doubt," he warned the terrorist group, "this...