The world's most powerful military alliance just reached its arm out to another nation

Montenegrin Prime Minister Milo Djukanovic will travel to Brussels on Wednesday to meet with NATO Secretary General Jens Stoltenberg and sign an accession agreement, the AFP reports.

The agreement will be the first step towards making Montenegro the 29th member of NATO.

"It is an historic day for the alliance, for Montenegro and for the stability of the Western Balkans," Stoltenberg told reporters in Brussels, the AFP notes.

"The enlargement of NATO with Montenegro confirms that NATO's door is open."

After signing the agreement, which is slated to take place on Thursday, Montenegro will be a party to NATO meetings, like July's coming Warsaw summit, as an observer until all 28 existing members ratify their membership, which could take up to 18 months.

NATO would be the third Balkan state to join NATO, joining Croatia and Romania.

NATO's expansion comes after Russia illegally annexed Crimea and used a form of hybrid warfare to destabilize Ukraine by backing anti-western rebels.

Russia has repeatedly threatened military actions to counter the buildup of NATO, but top NATO officials say that Russia will have absolutely no say in Montenegro's decision.

"The fundamental principle is that every nation has the right to decide its own path ... including to decide what security arrangements it wants to be part of," Slotenberg said.

"Any sanctions or reactions from Russia will be absolutely unjustified, because it's about respecting the sovereign decision of a sovereign nation, Montenegro, to decide on its own path and that should be respected by everyone," Stoltenberg continued.

Additionally, Georgia, Macedonia, and the Ukraine have expressed interest in joining NATO.