The North Atlantic Treaty Organization, also called the North Atlantic Alliance, is an intergovernmental military alliance between 29 North American and European countries. The organization implements the North Atlantic Treaty that was signed on 4 April 1949. NATO constitutes a system of collective defense whereby its independent member states agree to mutual defense in response to an attack by any external party. NATO's Headquarters are located in Evere, Brussels, Belgium, while the headquarters of Allied Command Operations is near Mons, Belgium. One of the most important issues reviewed by the NATO Member States, which will be discussed in our committee, “Question of Enlargement of NATO” has been a controversial topic. NATO membership is open to “any other European state in a position to further the principles of this Treaty and to contribute to the security of the North Atlantic area.” NATO also has what it calls the Membership Action Plan. It helps aspiring members prepare for membership and meet key requirements by providing practical advice and targeted assistance. Another agenda item that we have included in our committee is the supplement of military equipment from non-NATO member states. A recent example would be the S-400 acquisition of the Turkish Government from a non-NATO member state, the Russian Federation.