The exercise engaged 250 members of armed forces and civilian services from Croatia, the United States, Poland, Macedonia, and Albania.
Participants had to uncover a cargo of nuclear weapons of mass destruction which a group of international terrorists tried to smuggle through Croatia's territorial waters.
Representatives of Bosnia and Herzegovina, Serbia and Montenegro, and Slovenia were invited as observers.
The aims of the exercise, which ends on Friday, are to advance cooperation among the countries participating in ARIEX 06, to enable them to make timely estimates of the degree of danger from means of mass destruction and effectively react to such danger.
The exercise showed a high degree of cooperation of the many Croatian services which took part in it, it was assessed.
Foreign Affairs and European Integration Minister Kolinda Grabar-Kitarovic said Croatia had demonstrated with the exercise that it was ready for international cooperation, for responding to possible threats, and for developing coordination among its civilian and military services so as to react to such threats more effectively.
"Croatia expects full membership of NATO and the EU and wishes to strengthen regional security cooperation and stability," she said.
Defence Minister Roncevic said the exercise had demonstrated that the Croatian Armed Forces were also ready to deal with other tasks when international terrorism was involved.
US Ambassador Ralph Frank said he was satisfied with the results of the exercise, notably with the coordination of all participants who could effectively and promptly react to an international peace threat. He added the proliferation of means of mass destruction knew no boundaries but that good regional cooperation could effectively respond to such threats.