The signing was conducted during a ceremony at the alliance's headquarters in Brussels on Wednesday, July 9. It was largely a formality since NATO had agreed to bring the two Balkan countries into the fold at a summit in Bucharest in April.
"This is a historical achievement, not just for these two countries but for the entire Atlantic community of nations," said NATO Secretary-General Jaap de Hoop Scheffer. "Albania and Croatia have implemented major political and military reforms and therefore earned their rightful place at our table."
The protocols, signed by the ambassadors of the NATO member states in the presence of the Albanian and Croatian foreign ministers, alter the North Atlantic Treaty -- NATO's founding document -- to add the two Adriatic states' names to the list of members.
NATO accession is "the fulfilment of the paramount ambition of the Albanian people," and the ceremony "marks the crowning of our long and determined efforts," Albanian Foreign Minister Lulzim Basha said.
"This is a historical moment for Croatia: Joining the alliance represents one of the most important moments in our history," Croatian Foreign Minister Gordan Jandrokovic said. "This is the final chapter of a long process for Croatia and Albania ... a unique privilege for our generation."
However, one other country wishing to join the club, Macedonia, was not at the ceremony since its accession was blocked by Greece due to a long-running dispute over its name.
NATO's current members will now have to ratify the protocols in their national parliaments. Once they have done so, Albania and Croatia will have to ratify the modified version of the treaty, becoming full members as soon as they lodge the ratification documents in Washington.
NATO leaders invited the two Adriatic states to join their alliance at a summit in Bucharest in April.
Officials say that once the accession protocols have been signed, the ratification process usually takes approximately one year. During that period, the soon-to-be members have the right to take part in most NATO meetings, but they do not sit in alphabetical order among the current members and they have no role in decision making.
However, NATO leaders have indicated that they hope the process can be completed in time for the alliance's 60th anniversary in April 2009. The anniversary is set to be marked by a summit in the French town of Strasbourg and its German neighbor, Kehl.