NATO ministers have invited Montenegro to become the alliance's 29th member, the organization's first expansion in six years.
Foreign ministers extended the invitation to begin "accession talks" to Montenegro at a meeting in Brussels Wednesday.
Secretary General Jens Stoltenberg called the move a “historic decision” and said it affirmed NATO’s “open-door policy," Bloomberg reported.
Full membership for the former Yugoslav republic could take up to two years.
Montenegro's Prime Minister Milo Djukanovic said the invitation represents a historic day for the small state, the most important since their 2006 independence referendum, AP reported.
"Montenegro is entering the exclusive circle of states which are synonymous with the highest values of modern civilization," Djukanovic said.
"This is a crown jewel of the long-standing national efforts and comprehensive reform processes launched in 2006."
#Montenegro thanks @NATO allies for the invitation to join the Alliance.
Balkan states Croatia and Albania were the most recent countries to join the U.S.-led military alliance in 2009.
The move could anger Russia, which has stated its opposition to the plan before. Last year, Russia's Foreign Minister Sergei Lavrov said allowing Montenegro to join NATO would be a “provocation” and “irresponsible”, the Telegraph reported.