Some of this answer is from an earlier post on German unification.

The short answer is that Helmut Kohl's astute politicking was the main reason why both Gorbachev and the British and French, who were not entirely swayed by the allure of German reunification (a French postwar political aphorism was "we love Germany so much we want two of them!") went along with unification. As the Berlin Wall fell one of the facets that no diplomat was prepared for was the impotence of the SED leadership and the GDR's government. Simply put, Kohl was able to exploit this vacuum as being the the active German partner on the European stage. The Soviets' main German partner, the SED, was not able to position themselves as a major policy driver.

One of the major problems that faced the GDR was that it had to confront multiple crises in 1989 and its leadership was more reactive to events than proactive. The flight of tens of thousands of East Germans into West Germany created a crisis of legitimacy for the GDR. The out of control situation gave political reformers within the SED (the main political party of the GDR) ammunition to unseat its General Secretary Erich Honecker and replace him with Egon Krenz. Ironically, Krenz was Honecker’s own “crown prince” and was the Honecker’s own hand-picked successor. The problem was that Krenz had to deal with a series of broad-seated problems that were pervasive within the GDR and had a very limited time in which to accomplish these goals. The SED leadership in the Politburo had planned for a more organized and straight-forward series of gradual reforms, but events forced them to rush through and extemporize what was supposed to be a measured process that would retain the GDR as an independent political entity.

Adding further pressure to Krenz and the reformist SED was the increasing presence of the East German street calling for greater transparency, open elections, and a massive overhaul of the SED state. These demonstrations snowballed once it became clear that the 1989 was not going to repeat 1953, when the SED had sent in tanks to crush demonstrators. The general flat-footedness and incoherency of the post-Honecker SED is demonstrated by the series of decisions that led to the collapse of the Berlin Wall. On 9 Novemeber 1989, there was a live press conference held by the SED’s propaganda chief and Politburo member Günter Schabowski and an Italian journalist asked him a question on the new travel restrictions in the GDR. Schabowski, who was out of the loop for the Politburo meetings on travel restrictions, improvised an answer that asserted that the GDR was going to implement regulations allowing GDR citizens to leave any of its border crossings and when pressed, he indicated they would be immediately implemented. This led to a massive influx of Berliners congregating at the Wall and the border guards simply opening the gates when they could not receive any clear orders to do so.

The impotence of Krenz and the SED gave Helmut Kohl a golden opportunity to pursue unification and he and the CDU took it. This was actually not as easy of a process as one might assume. Both France and the UK were highly skeptical of German unification being within the larger interests of Europe. Part of Kohl’s canniness was that he recognized that the SED’s impotence gave him much greater latitude on the international stage. Kohl had developed a degree of contacts within both the USSR and the US and was able to sell both Bush and Gorbachev on the idea and popularity of German unification. Kohl gambled that if put to a vote, the citizens of the GDR would vote for unification. This gamble was correct and Kohl was able to present German unification as a fait accompli. Gorbachev had to recognize it in part because opposing it would be apparently standing against the tides of history and reform.

It should be noted that neither the SED nor the USSR ever intended for the GDR to dissolve and hoped that it had enough time to reform itself. Their perception was that the SED would be able to navigate into a "third-way" that would create a reformed socialist Germany. This was not an entirely illogical assumption. The idea of German unification had been something of a dead issue since Brandt’s Ostpolitik had formally established a degree of recognition to the GDR. Unification had to pass over the significant stumbling block of memory of the Third Reich and the spectre that German unification would lead to a renewed German drive for hegemony. This was a popular concern within most European states and built on both right-wing nationalist fears and some among Europe’s left-wing that perceived the CDU as possessing strong ideological continuities with the Third Reich (the logic here is that it Adenauer’s soft denazification policies had led to individual Nazis taking up positions of power within the FRG’s government and industry). Kohl was able to push for unification and twist or spin votes against the SED as a vote for a swift unification. While there was a sizable majority of East Germans who were for unification, there were quite a few East Germans who were also quite upset over their stagnant economy and hoped that a vote for Allianz bloc would economically integrate East Germany into the West and recreate the Economic Miracle of the Adenauer era.

Sources

Jarausch, Konrad Hugo. The Rush to German Unity. New York: Oxford University Press, 1994.

Kotkin, Stephen, and Jan Tomasz Gross. Uncivil Society 1989 and the Implosion of the Communist Establishment. New York: Modern Library, 2009.