The United States has informally told Japan it will retire its sea-based Tomahawk cruise missiles carrying nuclear warheads, in line with President Barack Obama's policy to pursue a world free of atomic weapons, government sources said Monday. Washington said the move would not affect the nuclear umbrella, addressing concerns in Tokyo about the step's effect on U.S. deterrence against potential attacks from China, North Korea or other countries, the sources said. The retirement will probably be stipulated in the Nuclear Posture Review, a new nuclear strategic guideline the Obama administration is slated to submit to Congress next month, they said....