NATO is a military alliance with the purpose of collective defense. NATO conducts military operations of two kinds: peace keeping and peace enforcing ones. The difference is that peace keeping actions are realized with the mutual agreement of  the parties involved. The ground base for interference in peace enforcing operations is a resolution of the United Nations security council. In reality, however, such operations have taken place without UN's approval: in 1995 NATO forces interfered in the conflict in Bosnia and Herzegovina with no UN approval, neither was sanctioned the 1999 NATO bombardment of Yugoslavia. The most widely known large-scale military operations of the NATO in this photo gallery by ITAR-TASS