Nuclear umbrella

The "nuclear umbrella" is a guarantee by a nuclear weapons state to defend a non-nuclear allied state. The context is usually the security alliances of the United States with Australia, Japan, South Korea, the North Atlantic Treaty Organization (much of Europe, Turkey and Canada) and the Compact of Free Association (the Federated States of Micronesia, the Marshall Islands and Palau). Those alliances were formed because of the Cold War and the Soviet Union. For some countries, it was an alternative to acquiring nuclear weapons themselves; other alternatives include regional nuclear-weapon-free zones or nuclear sharing.