West German Chancellor Helmut Kohl, on a diplomatic ice-breaking mission to the Soviet Union, laid a wreath at the tomb of the Soviet unknown soldier Tuesday and visited a cemetery where 596 German soldiers taken prisoner during World War II are buried. Kohl, who has vowed to forge ahead with his goal of a united Germany despite opposition from the Kremlin, was to meet again Tuesday with Soviet President Mikhail Gorbachev. Gorbachev said Monday after a 2 1/2-hour meeting with Kohl that the "ice has been broken" in relations between the two longtime antagonists, but cautioned that a continued push toward reunification was a dangerous policy for Bonn to pursue.