NATO: Pentagon's Gateway Into Former Warsaw Pact, Soviet Nations
The North Atlantic Treaty Organization was founded in April of 1949 by a country not on the European continent, the United States, and eleven subordinates which had fought on both sides of the World War that had ended four years earlier: Belgium, Britain, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway and Portugal. Greece and Turkey were added in 1952 after their service in the Korean War and West Germany joined in 1955.
Five days after the inclusion of the Federal Republic of Germany on May 9, in contravention of the 1945 Potsdam Agreement between Britain, the U.S. and the Soviet Union which explicitly demanded and meticulously detailed plans for the demilitarization of Germany, the Soviet Union established the Warsaw Treaty Organization (Warsaw Pact) in response. Fellow members were Albania, Bulgaria, Czechoslovakia, the German Democratic Republic (East Germany), Hungary, Poland and Romania. Albania formally withdrew in 1968, though it had not been a participating member since the early 1960s, and Romania had been a member in name only for at least twenty years before the pact's formal disbandment.
With the accession of Spain into the "military alliance of democratic states in Europe and North America" in 1982 the U.S.-led military bloc grew from its original 12 to 16 members. By that time the Warsaw Pact had shrunk from eight to seven members and some of the remaining ones were only selectively involved.
NATO had regularly conducted large-scale military exercises in alleged defense of Norway, Denmark and other members, but never deployed forces or conducted operations outside member states' territories, counting on the thousands of American nuclear warheads in European NATO states to respond to the Warsaw Pact's conventional military superiority in the event of armed confrontation. 
Military forces from the Warsaw Pact intervened in Czechoslovakia in 1968 and in the early 1980s it appeared they might do so again in Poland, and the Soviet Union sent troops to Hungary in 1956 after Prime Minister Imre Nagy withdrew his nation from the Warsaw Pact.
The Soviet Union's justification for those actions was that nations in Eastern Europe gravitating toward the West could be transformed into sites from which NATO, and especially its dominant member the U.S., would present a military threat on or near its borders.
In 1999, eight years after the formal dissolution of the Warsaw Pact and the fragmentation of the Soviet Union itself, the Czech Republic, Hungary and Poland were brought into NATO as full members during the bloc's fiftieth anniversary summit in Washington, DC, while NATO was conducting its first large-scale military operation outside the territory of its member states and its first major armed conflict: The almost three-month Operation Allied Force air war against Yugoslavia, which had not been a member of either Cold War military alliance.
The accession of three former Warsaw Pact nations in 1999 was the largest one-time expansion in NATO's history. Five years later at the Istanbul summit seven new members were inducted, six former Warsaw Pact countries, including three ex-Soviet republics, and a former federal republic of Yugoslavia: Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia.
With the earlier absorption of East Germany into NATO with German reunification in 1990, by 2004 every member of the erstwhile Warsaw Pact outside the Soviet Union except short-term member Albania had been brought into the Western military alliance. Albania was incorporated into NATO at the Strasbourg-Kehl summit last year.
The worst suspicions harbored east of the Cold War divide had been confirmed. Not only have all of the Soviet Union's previous allies in Eastern Europe been recruited into a Washington-dominated military bloc that for eleven years has been actively waging wars in Europe and beyond Europe in Asia, but territory of what had been the Soviet Union itself now contains a NATO air base (Lithuania) and a cyber warfare center (Estonia).
Once Soviet Republics like Ukraine, Georgia, Azerbaijan and Armenia are being actively pursued by NATO, which has held military exercises in those countries.
In 2007 NATO selected the Papa Air Base in Hungary for its first Strategic Airlift Capability (SAC) operation in support of the wars in Afghanistan and Iraq (for the present). In the same year the Alliance announced it would open its first training center in a former Warsaw Pact country, in the Polish city of Bydgoszcz.
Starting the year after they were admitted as full NATO members, Bulgaria and Romania were approached by the U.S. to offer the Pentagon access to several major military bases.  Both countries had turned their air bases over to Washington in late 2002 and early 2003 for the invasion of Iraq, but in 2005 and 2006 Secretary of State Condoleezza Rice signed formal agreements for the acquisition of military bases with Romania and Bulgaria, respectively. The Mihail Kogalniceanu Air Base and training and firing grounds in Babadag, Cincu and Smardan in Romania, and the Bezmer Air Base, the Graf Ignatievo Air Base and the Novo Selo Training Range in Bulgaria were locked into initial ten-year agreements. The Pentagon is not planning to leave, surely not after spending billions of dollars to modernize the facilities.
The deployment of between 5,000-10,000 U.S. troops to the bases at any one time is the first Pentagon presence in former Warsaw Pact nations. And the seven Bulgarian and Romanian installations are the first American military bases in any of those countries. Neither the troops nor the bases were the last.
The U.S. has moved its Joint Task Force-East, whose name alone indicates its purpose, to the Mihail Kogalniceanu Air Base in Romania, and in the words of the unit's deputy commander in 2008, "We are building a permanent forward operating station here...."