The START Treaty was negotiated by George W. Bush to allow the US to monitor Russia's missile development, but it expired last December. President Obama negotiated a "follow-on" called New START, to continue inspections and also reduce the number of missiles on both sides. Senate ratification is supported by US military leaders, NATO allies and former Republican Secretaries of State James Baker and Henry Kissinger. The President calls it "a national security imperative" in the lame-duck session. But key Republican leaders are saying, "Not so fast."