From the Introduction: "Poland has been a major focus of U.S. and NATO efforts to deter potential Russian aggression in Europe. This is due in part to its geographic location on NATO's eastern flank, providing land access to the Baltic States. Although Ukraine is not a NATO ally, the Russian occupation of Ukraine's Crimea region in 2014 and subsequent initiation of a separatist war in eastern Ukraine underscored to many 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 emphasize 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. The United States has bolstered security in Central and Eastern Europe with an increased rotational military presence, additional exercises and training with allies and partners, improved infrastructure to allow greater responsiveness, enhanced prepositioning of U.S. equipment, and intensified efforts to build partner capacity for newer NATO members and non-NATO countries. The European Deterrence Initiative (EDI), launched in 2014 (originally called the European Reassurance Initiative), an Overseas Contingency Operations account in the U.S. defense budget, is the key fiscal mechanism by which it does so. U.S. military operational activities of EDI are executed as part of Operation Atlantic Resolve (OAR). Approximately 6,000 U.S. military personnel are involved in OAR at any given time, with units typically operating in the region under a rotational nine-month deployment."
CRS In Focus, IF11280
Congressional Research Service: https://crsreports.congress.gov/