After the Cold War and NATO

The Revolutions of 1989 and the dissolution of the Warsaw Pact in 1991 removed the de facto main adversary of NATO and caused a strategic re-evaluation of NATO's purpose, nature, tasks, and focus on the continent of Europe.

The shift started, with the 1990 signing in Paris of the Treaty on Conventional Armed Forces in Europe between NATO and the Soviet Union, which mandated specific military reductions across the continent, which continued after the dissolution of the Soviet Union in December 1991. European countries then accounted for 34 per cent of NATO's military spending; by 2012, that had fallen to 21 percent.

NATO also began a gradual expansion to include newly autonomous countries of Central and Eastern Europe and extended its activities into political and humanitarian situations that had not been thought of as NATO concerns. An expansion of NATO came with German reunification on 3 October 1990, when former East Germany became part of the Federal Republic of Germany and of the alliance.

That had been agreed in the Two Plus Four Treaty earlier that year. To secure Soviet approval of a united Germany remaining in NATO, it was agreed that foreign troops and nuclear weapons would not be stationed in the east.

There was no formal commitment in the agreement not to expand NATO to the east, but there are diverging views on whether negotiators gave informal commitments regarding further NATO expansion. Jack Matlock, the American ambassador to the Soviet Union during its final years, said that the West gave a "clear commitment" not to expand, and declassified documents indicate that Soviet negotiators were given the impression that NATO membership was off the table for countries such as Czechoslovakia, Hungary, or Poland.

Hans-Dietrich Genscher, then the West German foreign minister, said in a conversation with Eduard Shevardnadze, "For us, however, one thing is certain: NATO will not expand to the east."

In 1996, Gorbachev wrote in his Memoirs that "during the negotiations on the unification of Germany they gave assurances that NATO would not extend its zone of operation to the east,"[48] and hr repeated that view in an interview in 2008.

However, in 2014 Gorbachev stated the opposite: "The topic of 'NATO expansion was not discussed at all [in 1990], and it wasn't brought up in those years. I say this with full responsibility. Western leaders didn't bring it up, either."

According to Robert Zoellick, a US State Department official involved in the Two Plus Four negotiating process, that appears to be a misperception, and no formal commitment regarding enlargement was made. Harvard University historian Mark Kramer also rejects that an informal agreement existed.

As part of the restructuring, NATO's military structure was cut back and reorganized, with new forces such as the Headquarters Allied Command Europe Rapid Reaction Corps established. The changes brought about by the collapse of the Soviet Union on the military balance in Europe were recognized in the Adapted Conventional Armed Forces in Europe Treaty, which was signed in 1999.

The policies of French President Nicolas Sarkozy resulted in a major reform of France's military position, culminating with the return to full membership on 4 April 2009, which also included France rejoining the NATO Military Command Structure but maintaining an independent nuclear deterrent.