An intergovernmental organisation charged with implementing the North Atlantic Treaty signed in 1949. NATO is centred upon the concept of collective defence in which an attack on one is considered to be an attack upon all. Members therefore accept the principle of mutual defence in response to an external attack. Since the end of the cold war, the organisation has sought to rebrand itself as a more humanitarian organisation. The organisation has also expanded into the Soviet sphere of influence, and has implemented military action in response to a unanimous position adopted by the UNSC (e.g. enforcing a no-fly zone over Libya in 2011).