What the Germans, Americans, British and French did agree to in 1990 was that there would be no deployment of non-German NATO forces on the territory of the former GDR. I was a deputy director on the State Department's Soviet desk at the time, and that was certainly the point of Secretary James Baker's discussions with Gorbachev and his foreign minister, Eduard Shevardnadze. In 1990, few gave the possibility of a broader NATO enlargement to the east any serious thought.
The agreement on not deploying foreign troops on the territory of the former GDR was incorporated in
Article 5 of the Treaty on the Final Settlement with Respect to Germany, which was signed on September 12, 1990 by the foreign ministers of the two Germanys, the United States, Soviet Union, Britain and France. Article 5 had three provisions:
....
Russia behind the Headlines has published an
interview with Gorbachev, who was Soviet president during the discussions and treaty negotiations concerning German reunification. The interviewer asked why Gorbachev did not "insist that the promises made to you [Gorbachev]—particularly U.S. Secretary of State James Baker's promise that NATO would not expand into the East—be legally encoded?" Gorbachev replied: "The topic of 'NATO expansion' was not discussed at all, and it wasn't brought up in those years. … Another issue we brought up was discussed: making sure that NATO's military structures would not advance and that additional armed forces would not be deployed on the territory of the then-GDR after German reunification. Baker's statement was made in that context… Everything that could have been and needed to be done to solidify that political obligation was done. And fulfilled."
"Gorbachev continued that "The agreement on a final settlement with Germany said that no new military structures would be created in the eastern part of the country; no additional troops would be deployed; no weapons of mass destruction would be placed there. It has been obeyed all these years." To be sure, the former Soviet president criticized NATO enlargement and called it a violation of the spirit of the assurances given Moscow in 1990, but he made clear there was no promise regarding broader enlargement.