Membership with the North Atlantic Treaty Organization (NATO).
Explanation:
The North Atlantic Treaty Organization, also called the North Atlantic Alliance, is an intergovernmental military alliance. Currently, 29 North American and European countries are members of NATO. This organization is actually a collective defense system in which the countries involved agree to mutual defense as a response to any attack by an external party.
States and the federal government have both exclusive powers and concurrent powers. There is an ongoing negotiation over the balance of power between the two levels.
