From the Summary: "In less than a year, East Germany underwent a remarkable political transformation. A combination of massive emigration to the West and huge street demonstrations in cities throughout East Germany led to the collapse of the hard-line regime of Erich Honecker on October 18, 1989. The opening of the border between the two Germanys on November 9 and continuing deep public mistrust of the government provoked an accelerated exodus of East Germans to the West. Political debate in the country quickly shifted from how to reform East Germany to how and when East Germany should be reunited with West Germany. […] In February 1990, West Germany agreed to a 'Two-plus-Four' (the two Germanys and the four victorious allies from World War II) formula for discussing the international ramifications of unification. On September 12, 1990, these talks yielded a treaty restoring full sovereignty to a united Germany. Among the principal provisions of the treaty were agreement that a united Germany would remain in NATO, would make no future border claims, and would remain a non-nuclear state. In addition, in the event of a conventional arms agreement for Europe, Germany will radically reduce its armed forces, and Soviet forces will withdraw from eastern Germany by the end of 1994. The President sent the Treaty to the U.S. Senate on September 26, 1990. On October 10, the Senate gave its advice and consent to the Treaty."
CRS Report for Congress, 90-523 F