The following is the April 7, 2021 Congressional Research Service Insight Report, The Open Skies Treaty: Background and Issues.
From the report
The United States announced its withdrawal from the Treaty on Open Skies on May 22, 2020; this withdrawal took effect on November 22, 2020. The United States no longer participates in flights or shares data collected by others. Russia also announced its plans to withdraw, while beginning the domestic procedures for withdrawal on January 15, 2021. It has indicated that it will soon send its official notification to the other treaty parties.
The United States, Canada, and 22 European nations signed this treaty on March 24, 1992. It entered into force on January 1, 2002, and had 34 members before the U.S. withdrawal. The parties permit unarmed observation aircraft to fly over their entire territories to observe military forces and activities. The treaty is designed to increase transparency, build confidence, and encourage cooperation among European nations.
The parties conducted 1,500 observation flights through October 2019. Some parties provide their own aircraft, but they can also join overflights on aircraft provided by other nations. Both the observing nation and observed nation have access to the data from each flight; other parties can purchase the data, so all can share information collected during all flights. According to the State Department, the United States conducted nearly three times as many flights over Russia as Russia did over the United States. Further, the parties can invite flights over their territories in special circumstances, as Ukraine did in 2014, when Open Skies flights helped monitor activities along the Ukraine-Russian border.
Russian officials insisted that the remaining participants not share data collected on Open Skies flights with the United States and indicated that they expected to continue to fly over U.S. bases and facilities located on the territories of other treaty parties. After the other parties rejected these conditions, Russia announced its plans to withdraw from the treaty.
The Biden Administration has not yet decided whether it will seek to rejoin the treaty, but recent reports indicate the Administration has informed U.S. allies that it is concerned that rejoining the treaty could “send the wrong message to Russia and undermine our position on the broader arms control agenda” if Russia continues to violate Open Skies. In addition, the Air Force has removed the U.S. Open Skies aircraft from service and has taken steps to retire them.
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Source: USNI News