A nuclear arms treaty limiting the number of atomic warheads the United States and Russia are allowed to possess, is now in effect.
Russian Foreign Minister Sergei Lavrov and the US Secretary of State Hillary Clinton exchanged ratification documents at a conference in Munich on Saturday.
The treaty replaces the 1991 START treaty, which expired in December 2009.
It was approved by the US Senate in December and by the Russian parliament last month.
The papers exchanged in Munich were signed by US President Barack Obama and Russian President Dmitry Medvedev following an agreement made last April.
The BBC reports the new treaty limits each side to no more than 800 deployed nuclear warhead delivery systems including bombers, missile launchers and nuclear submarines - a cut of about 50%.
It limits each side to 1,550 deployed warheads.