[Lurking Libertarian]

This 1974 Treaty was with a country called the U.S.S.R. The U.S.S.R. no longer exists. How can we have a treaty with a country that doesn't exists?

There are a number of problems I have with this suit, but this is not one of them. There is a well-established doctrine in international law called "state succession" which keeps countries from getting out of treaties by changing their government. The US successfully used this doctrine to keep the USSR from getting out of treaties signed by Czarist Russia, and it is equally applicable now to bind Russia to treaties signed by the USSR.

[ExpandNATO]

There is a well-established doctrine in international law called "state succession" which keeps countries from getting out of treaties by changing their government.

From Douglas Feith of Feith and Zell, P.C. (law firm). Quote from Center for Security Policy

Furthermore, the newly independent states that arose on the USSR's territory jointly declared at Minsk on December 8, 1991, that none of them preserved the legal personality of the Soviet Union, which 'as the subject of international law and geopolitical reality no longer exists.' The U.S. Government promptly thereafter acknowledged that the Soviet Union had dissolved.

Furthermore, just try to get Russia to assume any obligation or pay damages resulting from the Soviet Union's activities. Now if they can frustrate American ABM development all of a sudden Russia is the successor. The treaty died the day the Soviet Union did.

[Medium Rare]

The US successfully used this doctrine to keep the USSR from getting out of treaties signed by Czarist Russia, and it is equally applicable now to bind Russia to treaties signed by the USSR.

That was a succession of governments in the same country. The dissolution of the USSR was more than a change of government, it dissolved a country.