Washington ? President Barack Obama is taking the first major step in his push toward a nuclear-free world, returning to Prague to sign the kind of arms-reduction treaty with Russia unseen for nearly two decades.

The deal goes beyond modest arsenal reductions, offering Obama a chance to repair soured relations with Moscow and pursue more dramatic cuts in global nuclear weapon stockpiles.

The new treaty, to be signed today by Obama and Russian President Dmitry Medvedev, will shrink both nations’ arsenals of strategic nuclear warheads to 1,550 over seven years, about a third less than the 2,200 currently permitted. It was a year ago nearly to the day, also in Prague, that Obama outlined his agenda to reduce the role of nuclear weapons in U.S. security strategy, with a long-range goal of eliminating nuclear arms.