Normally one would think Afghanistan, out-of-area deployment, mobility and interoperability when discussing the problems facing NATO in the 21st Century.  Well, the latest problem creeping up on the NATO plate is Slovenia-Croatia bilateral relations. 

They are not good; let’s start with that.
Slovenia, a NATO member, may hold a general referendum on whether Croatia should be allowed into the alliance. The opposition party is demanding it. 
Croatia was invited to join NATO, together with Albania, at the Bucharest Summit last April. It was a milestone achievement when considering that a decade ago the Balkan region was in a war.
Macedonia didn’t clear the Bucharest hurdle because of its bilateral problems with Greece.  Athens disputes Macedonia’s name, arguing that Macedonia is a province in Greece. Thus, Macedonia was the first to get nixed off the Adriatic Charter list, which was originally made up of Macedonia, Albania and Croatia.  The original idea behind the Adriatic Charter was to sell the Balkan trio in a package to NATO, thus increasing the collective strategic relevance of the NATO enlargement to the Balkans, and increasing each country’s chance to get in.
Slovenia and Croatia have multiple outstanding disagreements, with the right to international waters for Slovenia being the most acute problem. A member of both NATO and the EU, Slovenia is looking to use its position to squeeze a compromise out of Zagreb. This is good realpolitik, and it probably makes sense in some very narrow way, but it is irritating Slovenia’s international partners and jeopardizing the country’s international standing which Ljubljana worked so hard to establish – through EU Presidency, OSCE Chairmanship and the UN Security Council seat.
The referendum idea is outright stupid. If held it will have nothing do with international security, NATO or Croatia’s readiness to be an equal partner in the alliance.  Therefore the alliance should send a clear and strong message to Slovenia not to turn Croatia’s membership into provincial populism, and thus risk compromising the broader strategic interests of the alliance. After all, keeping NATO enlargement on track goes well beyond Croatia and the Balkans. It will send a message to the rest of the aspirants that NATO is still a serious club worth joining, and most importantly, that the doors to NATO remain open for those able and willing to meet NATO standards and operate by NATO rules.   

Borut Grgic is a nonresident senior fellow at the Atlantic Council.