Macedonia and Montenegro should be invited to join NATO at next year’s Warsaw, Poland, summit, said Polish defense minister Tomasz Siemoniak at a conference in the Polish city Wroclaw on Friday. The two former Yugoslav nations want to join the 28-country military alliance, but any move to do so could increase already-high tensions between the Western alliance and Russia.

"It would be excellent news if the invitations could be sent from (the NATO summit in) Warsaw to Macedonia and Montenegro," said Siemoniak. "It seems that the NATO summit in Warsaw, if deprived of this element, will not bring full satisfaction to many nations, including Poland."

But even an invitation is likely to draw scorn from Moscow. Since the end of the Cold War, Russia has opposed any expansion of NATO that includes the former communist nations in eastern and southeast Europe, claiming it is a purposely provocative move. Russia's foreign minister has warned against NATO approaching Bosnia, Macedonia and Montenegro in the past, saying that NATO's allowing those countries to join would only serve to undermine Russia.  

This type of disagreement -- asking countries to choose allegiance to either the West or the East -- was the ideological barrier that fueled the Cold War for more than 40 years and lies at the heart of the current conflict in eastern Ukraine. Some believe that the war in the contested region of Donbas, Ukraine, is designed to stop the country from being eligible for NATO selection, as the alliance does not typically allow nations to join while a conflict remains unresolved. Experts say this tactic, known as a “frozen conflict,” was used in the 2008 war in Georgia.

In 1999, former communist countries began joining NATO en masse, including the former Soviet states of Estonia, Latvia and Lithuania, who all joined in 2004. In the Balkan region, Albania, Bulgaria, Croatia, Greece, Slovenia and Romania are members of the alliance.