Slovenia had delayed ratification of Croatian membership over a border dispute. This concerns the Bay of Piran and dates back many years. Croatia claims half the bay, a claim disputed by Slovenia which says that if Croatia gets it way then Slovenia's access to the Adriatic Sea will be blocked. (The BBC has a good report on this, and the map above comes from that story.)
Slovenian nationalists failed to collect enough signatures to force a referendum by the March 26 deadline, and on Friday the President confirmed the Parliament's ratification vote dating back to February. To the relief of NATO officials, Slovenia deposited its instrument of ratification with the United States on Monday. So, Croatia will officially join NATO this weekend.
Meanwhile, the territorial dispute continues. Slovenia is still blocking EU membership for Croatia. EU Commissioner on expansion Olli Rehn tabled a new compromise proposal last week, which the two parties are studying.
But for NATO the problem is over, and one cloud hanging over the Summit has evaporated.