National Nuclear Risk Reduction Center

The Trump administration notified international partners on May 21 that it's pulling out of a treaty that permits 30-plus nations to conduct unarmed, observation flights over each other's territory overflights initially set up to promote trust and avert conflict between the U.S. and Russia.

Russia has threatened to retaliate against the US in a row over the Open Skies Treaty that allows military observation flights over each other's territory.

Previously, the US accused Russia of violating the agreement, and threatened to take measures against Moscow.