Russia has announced it's leaving the Open Skies treaty, after the US exited late last year.
The treaty allows armed forces to fly over the territory of other signatories to gather reconnaissance, as part of an effort to increase transparency and predictability between countries.
But with the US gone, Russia sees little reason to remain.
Stuart Smith reports from Moscow.
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It allowed the International community to verify that Russia did not have a build-up of forces on the border by flying overheard with reconnaissance planes.
The US had traditionally flown far more flights over Russia than Russia did over the US.
But America pulled out after Republican senators said it was a strategic disadvantage to the US.
Russia's withdrawal means it'll be harder for it to monitor NATO troops in Europe, or military movements over the United States.
And with its two biggest players gone, the value of the treaty itself is in question.
Stuart Smith, Moscow