President Trump announced on January 17 that the United States will seek to develop new technologies to enhance its missile defenses in order to detect and intercept an adversary's missiles, including enemy missiles still on the ground in their boost phase prior to launch, or moments after liftoff. The announcement warrants a review of the law of nuclear weapons.

The United States is presently a party to 10 nuclear arms control agreements. Four of them prevent the United States from deploying nuclear weapons in specific geographical areas: The Antarctic Treaty; The Outer Space Treaty; The Seabed Arms Control Treaty; and The Treaty of Tiateloco which covers Latin America and the Caribbean.

Five agreements limit either:

(1) the testing of nuclear weapons (The Limited Test Ban Treaty prohibits tests in the atmosphere, in outer space, and under water; and, The Threshold Test Ban Treaty together with The Peaceful Nuclear Explosions Treaty regulate all underground nuclear explosions and prohibits such tests having a yield above 150,000 tons of TNT);

(2) the transfer of nuclear weapons (The Nuclear Non-Proliferation Treaty); or

(3) the number of strategic deployed nuclear warheads and strategic delivery systems (The New START Treaty limits each country to 1550 strategic nuclear warheads on 700 strategic delivery systems).

Finally, the Intermediate-Range Nuclear Forces Treaty, which came into effect in 1988, requires the United States and Russia to eliminate all their nuclear and conventional ground-launched ballistic and cruise missiles with ranges of 500 to 5,500 kilometers. (The United States has given notice to Russia that it intends to withdraw from the Treaty on the grounds that Russia is violating it and China is not a signatory. Pursuant to the Treaty's terms, the U.S withdrawal would take effect about mid-August 2019 at the earliest.)

The fundamental unwritten law governing nuclear weapons relations between the United States and Russia is the maintenance of mutual security. (While China is expanding its offensive nuclear systems, it seems to be giving more emphasis to developing a very robust conventional weapons capability.) Known as Mutual Assured Destruction (to some the strategy is truly MAD), the strategy rests on both countries knowing they cannot start a nuclear war because they both know they will lose. Both countries know this because each maintains offensive nuclear forces which can survive sufficiently a surprise first strike and then launch a second retaliatory strike which would impose unacceptable damage on the aggressor. It was for this reason that the very first bilateral strategic nuclear agreement between the United States and the then Soviet Union, the Strategic Arms Limitation Treaty (SALT I) ratified by both in 1972, limited strategic missile defenses to only 200 interceptors at two sites. SALT I ensured both countries they could maintain their devastating second retaliatory strike capability because they knew they could overwhelm the other's limited anti-ballistic missile defenses.

In sum, the United States and Russia are governed, metaphorically, by Isaac Newton's (1643-1727) Third Law, which states that for every action there is an equal and opposite reaction. In strategic nuclear terms, this means that every new nuclear weapons development either the United States or Russia takes that has the potential of upsetting mutual security will provoke an opposite reaction of equal force by the other side. Just as much as the United States would take all steps necessary to ensure its offensive missiles could penetrate any new Russian defensive system, the President's missile defense announcement will stimulate a Russian policy to ensure its offensive nuclear forces have the capability of penetrating any enhanced U.S. defense. (This is true even if the U.S. defensive system is focused on destroying small numbers of missiles launched by regional powers, such as North Korea. Russia, like the United States, will always plan on what is called “worst-case scenario.”)

Relations today between the United States and Russia recall Cold War visions of nuclear adversaries. Arms control discussions appear to be non-existent. This is dangerous. The START Treaty will expire in 2021, though both parties by Treaty terms may agree to extend it for a period of up to five years. As difficult as it may be, it is imperative to separate arms control issues from all other bilateral issues. The more fragile the overall relationship, the more existential it is to maintain mutual nuclear security.

David Lenefsky practices law in Manhattan. He was formerly Project Director for Arms Control at The United Nations Association—USA.