It wasn't just the James Baker comment...
Many world leaders made certain assurances:
The first concrete assurances by Western leaders on NATO began on January 31, 1990, when West German Foreign Minister Hans-Dietrich Genscher opened the bidding with a major public speech at Tutzing, in Bavaria, on German unification. The U.S. Embassy in Bonn informed Washington that Genscher made clear “that the changes in Eastern Europe and the German unification process must not lead to an ‘impairment of Soviet security interests.’ Therefore, NATO should rule out an ‘expansion of its territory towards the east, i.e. moving it closer to the Soviet borders.’”
The conversations before Kohl’s assurance involved explicit discussion of NATO expansion, the Central and East European countries, and how to convince the Soviets to accept unification. For example, on February 6, 1990, when Genscher met with British Foreign Minister Douglas Hurd, the British record showed Genscher saying, “The Russians must have some assurance that if, for example, the Polish Government left the Warsaw Pact one day, they would not join NATO the next.”
Baker said it again, directly to Gorbachev on May 18, 1990 in Moscow, giving Gorbachev his “nine points,” which included the transformation of NATO, strengthening European structures, keeping Germany non-nuclear, and taking Soviet security interests into account. Baker started off his remarks, “Before saying a few words about the German issue, I wanted to emphasize that our policies are not aimed at separating Eastern Europe from the Soviet Union. We had that policy before. But today we are interested in building a stable Europe, and doing it together with you.”
The documents show that Gorbachev agreed to German unification in NATO as the result of this cascade of assurances, and on the basis of his own analysis that the future of the Soviet Union depended on its integration into Europe, for which Germany would be the decisive actor. He and most of his allies believed that some version of the common European home was still possible and would develop alongside the transformation of NATO to lead to a more inclusive and integrated European space, that the post-Cold War settlement would take account of the Soviet security interests. The alliance with Germany would not only overcome the Cold War but also turn on its head the legacy of the Great Patriotic War.
But inside the U.S. government, a different discussion continued, a debate about relations between NATO and Eastern Europe. Opinions differed, but the suggestion from the Defense Department as of October 25, 1990 was to leave “the door ajar” for East European membership in NATO.