President Obama landed in Prague this morning to sign a treaty with Russia to reduce nuclear weapons in a historic move for the former Cold War foes that marks a long-overdue diplomatic success for the American leader. The ceremony at Prague Castle, the site of a speech by Mr Obama a year ago about his desire for a world without nuclear bombs, also reflects the "resetting" of relations between Washington and Moscow and places Russia at the heart of future attempts to encourage other states to ditch atomic warheads.The new Strategic Arms Reduction Treaty (Start) — already delayed because of difficulties in negotiations - could yet be undermined, however.. .