Sen. John McCain is waging another national campaign—this time, to define his legacy. After two unsuccessful presidential bids, the 78-year-old former Navy pilot and Vietnam prisoner of war has rebounded as the chairman of the powerful Senate Armed Services Committee. The post gives the Arizona Republican a significant say on national security—and a chance to ensure that his loss to Barack Obama in the 2008 White House race isn’t the final word in the colorful McCain chronicles. McCain wants to prod the Obama administration, which he derides as feckless, to adopt a tougher policy against worldwide threats. He wants budget...