From the Introduction: "Poland has been a major focus of U.S. and NATO efforts to deter potential Russian aggression in Europe. This is in part due to its geographic location situated on NATO's eastern flank, providing land access to the Baltic States. Although Ukraine is not a NATO ally, the Russian annexation of Crimea in 2014 and subsequent initiation of a proxy war in Eastern Ukraine underscored to many international security observers that NATO allies, particularly those in Eastern Europe, could once again be threatened by Moscow. In response, the United States and its NATO allies have undertaken a number of initiatives to underscore NATO's collective defense agreements, thereby assuring allies of their own security while simultaneously deterring Russian aggression. Poland is a critical partner in these efforts."
CRS In Focus, IF11280
Congressional Research Service: https://crsreports.congress.gov/