The following text aims to clarify the concept of a ‘nuclear umbrella’ and pinpoints political challenges and debates associated with the topic.
The euphemism of a ‘nuclear umbrella’ is generally understood to cover a form of military cooperation by which one or more nuclear-armed states (‘holding’ the umbrella) provide supposed nuclear protection for one or more non-nuclear-armed states (‘sheltering’ under it). A crucial point to understand about nuclear umbrellas is that they are not necessarily codified by authoritative documents. Rather, nuclear umbrellas are rooted in military and diplomatic practices. A ‘nuclear umbrella’, is a security arrangement under which the participating states consent to or acquiesce the potential use of nuclear weapons in their defence. The related concept of ‘extended nuclear deterrence’ may be understood as the intended effect of a nuclear umbrella. A ‘nuclear umbrella state’ is a state without nuclear weapons under the supposed protection of the nuclear weapons of another state.
Thus, while the Western nuclear-armed states’ nuclear umbrella over NATO is formalised in the alliance’s Strategic Concept (adopted by the member states’ heads of state), the United States’ nuclear umbrella over Australia is not codified in any written document. In fact, ‘there is no known United States official statement specifically providing an assurance of American nuclear protection for Australia in the face of nuclear threat or nuclear attack.’ Nevertheless, owing, in part, to the statements of Australian officials, there seems to be a general understanding among policy analysts that Australia is indeed covered by the US’ nuclear umbrella. We may thus distinguish between ‘formal’ (i.e. officially and jointly proclaimed) and ‘informal’ (i.e. based on unofficial common understandings) nuclear umbrellas. While NATO and the United States–South Korea security cooperation instantiate formal umbrellas, the United States–Japan security cooperation, the United States–Australia cooperation, and Russia’s nuclear umbrella over the Collective Security Treaty Organization exemplify informal umbrellas.
A nuclear umbrella is not simply a by-product of a military alliance involving nuclear- and non-nuclear states. In order to exist, a nuclear umbrella must both be contended and not explicitly refuted. Someone must declare that a relationship of extended nuclear deterrence is in operation, and then the other party (or parties) must accept that statement to be correct—either tacitly or explicitly. Egypt, for example, a military ally of the United States, forcefully rejected the United States’ alleged offer of extending its nuclear umbrella over Egypt in 2009. Several other states, including Argentina, Syria, and the Philippines are allied to nuclear-armed states without being considered umbrella states for that reason. Nuclear umbrellas must be mutually accepted.
The practice of extending nuclear umbrellas has caused considerable controversy, as it arguably undermines efforts at nuclear non-proliferation and disarmament. By subscribing to supposed nuclear protection, umbrella states signal that nuclear weapons are useful tools to enhance national security. Another controversy has been over certain states’ simultaneous adherence to a nuclear-weapon-free-zone treaty and a nuclear umbrella. On the definition above, four countries are in this situation per 2015: While Kazakhstan, Kyrgyzstan, and Tajikistan are simultaneously members of CSTO and parties to the Central Asian Nuclear-Weapon-Free Zone Treaty, Australia is simultaneously under the US’ nuclear umbrella and a party to the South Pacific Nuclear-Weapon-Free Zone.
Even more controversial is the practice of ‘nuclear sharing’, whereby one or more non-nuclear-armed states are engaged in the nuclear planning of one or more nuclear-armed states. This is standard practice in NATO, where all member states bar France are members of the Nuclear Planning Group. The United States has also long engaged in a practice of stationing nuclear weapons in allied countries. Nuclear warheads are still hosted by five NATO member states: Belgium, Germany, Italy, the Netherlands, and Turkey. American weapons were previously also stationed in the NATO members Canada, Greece, and the United Kingdom, and in (non-NATO members) the Philippines, South Korea, and Taiwan. The practice of stationing nuclear weapons in other countries has been sharply criticised by a number of states. They point out that Article I of the NPT commits the five recognised nuclear-weapon states (China, France, Russia, the United Kingdom, and the United States) not to transfer nuclear weapons or technology to non-nuclear-weapon states. The corollary Article II commits non-nuclear weapon states not to receive nuclear weapons.
In response to claims that foreign nuclear hosting violates the letter and spirit of the NPT, the United States and its allies claim that since US forces control the weapons even when they are stationed abroad, there can be no talk of nuclear weapons having been ‘transferred’ to another state. Another line of defence has been that even if the nuclear weapons that are placed in Europe may be carried and released by planes belonging to their host countries, this would only happen in a situation of war, in which case the NPT would no longer be binding. However, the final document of the 1985 Third Review Conference of the NPT established that the treaty remains in force under ‘any circumstances’, hence also in times of war.