While it might currently be challenging to imagine Russia as a NATO member, a change to the future strategic context could make it equally difficult to imagine Russia outside of NATO. The analysis of previous rounds of NATO enlargement informs the understanding of the potential operational consequences associated with the addition of Russia into the Alliance, however Russia also presents unique considerations that must be assessed prior to extending the invitation to Russia to join NATO. The idea of having Russia join NATO first surfaced in the post-Cold War period in a 1991 letter from President Boris Yeltsin to NATO. In this letter, Yeltsin stated that Alliance membership was a long-term Russian political aim. More recently, in 2009 the Polish Foreign Minister Rados aw Sikorski publicly voiced the idea of inviting Russia to join NATO. In 2010, several influential German foreign policy experts wrote an open letter arguing in favor of inviting Russia to join NATO in the widely read German weekly newsmagazine, Der Spiegel. This monograph explores the potential operational consequences for the planning and conduct of operations associated with Russia becoming a NATO member. NATO was created in 1949 as a collective defense alliance between twelve Western European and North American countries to counter Soviet influence. It has endured over sixty years and has survived the collapse of the Soviet Union, which was the threat that prompted its creation. During that time, NATO has grown to twenty-eight member states through six rounds of expansion: 1952, 1955, 1982, 1999, 2004, and 2009. The 'open door' concept for expansion is a fundamental component of the Alliance and was included in the original North Atlantic Treaty. Article 10 of the 1949 Washington Treaty promulgates the 'open door' concept by stating that the Alliance remains open to new members provided they are in a position to further the principles of the Treaty and contribute to the collective defense of the North Atlantic Area. Expansion has therefore been an inherent component to the evolution of the Alliance. Indeed, NATO has repeatedly adapted to the external strategic environment or the internal constraints and desires of member states. The most significant external security environment changes included the end of the Cold War and the post-2001 focus on counter-terrorism. Internal constraints and desires include, but are not limited to the concept of burdensharing, ideas regarding the Alliance strategic concept, and the international relations of individual member states. However, dealing with the Soviet Union and, since 1991, with Russia has challenged NATO since its inception.