From the Document: "According to Secretary of State Michael Pompeo, the United States will give notice of its intent to withdraw from the Treaty on Open Skies on May 22, 2020; the withdrawal will occur in six months, on November 22, 2020. The United States, Canada, and 22 European nations signed this treaty on March 24, 1992. It entered into force on January 1, 2002, and now has 34 members. 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 had conducted 1,500 observation flights through early 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 copies of the data, so all can share information collected during all flights."
CRS Insight, IN10502
Congressional Research Service: https://crsreports.congress.gov/