Among contemporary alliances the North Atlantic Treaty Organization (NATO) is not only the largest and most active but particularly interesting from the point of view of relations among big powers. NATO has evolved from a defense alliance deterring or defending against aggression into an institution projecting security beyond its borders by stabilizing areas from which a threat to its members could emanate but also wherever the local situation creates unacceptable suffering. NATO notably plays this role in the Balkans and Afghanistan. Combining civilian and military measures, it simultaneously prevents conflicts from re-emerging and rebuilds institutions, the rule of law, civil society, economy and democracy quasi under a military roof, thus addressing the roots of conflict and of terrorism.