From the Document: "The United States announced its intention to withdraw from the Treaty on Open Skies on May 22, 2020; this withdrawal took effect on November 22, 2020. Russia has also announced its plans to withdraw, noting that it would begin the domestic procedures for withdrawal on January 15, 2021, and would then send the 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. [...] With the United States withdrawal from the Treaty, it will no longer participate in flights or share data collected by others. Russian officials insisted that the remaining participants not share data collected on Open Skies flights with the United States. They also 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."
CRS Insight, IN10502
Congressional Research Service: https://crsreports.congress.gov/