"Despite the grants of general sovereignty to both German states in 1955, full and unrestricted sovereignty under international law was not enjoyed by any German government until after the reunification of Germany in October 1990. Though West Germany was effectively independent, the western Allies maintained limited legal jurisdiction over 'Germany as a whole' in respect of West Germany and Berlin. At the same time, East Germany progressed from being a satellite state of the Soviet Union to increasing independence of action; while still deferring in matters of security to Soviet authority. The provisions of the Treaty on the Final Settlement with Respect to Germany, also known as the "Two-plus-Four Treaty", granting full sovereign powers to Germany did not become law until 15 March 1991, after all of the participating governments had ratified the treaty. As envisaged by the Treaty, the last occupation troops departed from Germany when the Russian presence was terminated in 1994, although the Belgian Forces in Germany stayed in German territory until the end of 2005. "
(from
Wikipedia
So East Germany was in fact a Soviet satellite state, and the Russians had a large military presence there.
The Western allies also had significant forces in Germany.
Now 'm not saying they're going to war immediately, but the diplomatic response will be "No, you have to accept to Oder-Neisse border, if you don't you're not going to be reunified, and according to the current treaties you can't." Which BTW is basically what was codified in OTL's
treaty on the final settlement of Germany. Germany agreed on the Oder-Neisse border in that treaty. If that treaty isn't signed, the allies won't grant Germany it's final sovereignity. Germany can resist and reunify regardless. In the worst case it escalates to war. In the best case Germany is an international pariah.