NATO foreign ministers on Wednesday invited Montenegro to join the military alliance as its 29th member.
NATO Secretary-General Jens Stoltenberg congratulated the country and said, "This is the beginning of a very beautiful alliance."
— NATO (@NATO) December 2, 2015
The accession period before Montenegro becomes a member is expected to take a year.
The military alliance bombed the country during the Kosovo war 16 years ago, when it was a constituent republic of Yugoslavia.
The military alliance last expanded six years ago with the joining of Albania and Croatia.
Russia, which has close economic ties with Montenegro, has repeatedly objected to the country joining NATO and has also said it would consider such a move a "provocation," and a threat to stability in the western Balkans.
Following NATO's invitation to Montenegro, Russia announced it will end joint projects with Montenegro if it were to join the alliance.
Tensions between NATO and Russia have been rising since Moscow unilaterally annexed Crimea and has given support to separatist forces in eastern Ukraine.
Stoltenberg reiterated NATO’s stance towards the Ukrainian crisis and said that the annexation of Crimea was "an act of aggression" by Moscow.
After the fall of Soviet Union NATO began to integrate former Soviet members and allies, such as Poland, Latvia and Lithuania to the alliance.
The Kremlin has accused NATO of militarising its member states in Eastern Europe and the Baltic countries neighbouring Russia, thereby upsetting the strategic balance of power around its borders.
In late June, United States Defense Secretary Ashton Carter told reporters with the defence chiefs that several countries in Eastern Europe and the Baltics - including Estonia, Lithuania, Latvia as well as Bulgaria, Romania and Poland - had agreed to deploy US-led NATO armaments.
However, Stoltenberg denied that the Euro-Atlantic alliance would resume a Cold War style arms race with Russia, but only boost its defence capabilities in order to reassure the security of its member states neighbouring Moscow.
In response to NATO’s reinforcement of its defensive capabilities Russia said it will enhance its nuclear arsenal by adding more than 40 new intercontinental ballistic missiles by the end of 2015.