ONCE GERMAN reunification swerved onto the express track, logic and history demanded that the new Germany be permanently fastened in NATO and not permitted to float out under some separate or neutralist flag. This was always the best way to guarantee the rest of Europe against the specter of a powerful, revived, maneuvering Germany. Naturally, the Soviet Union sought the best terms available for a step that was bound to be politically and emotionally traumatic for it. This step President Mikhail Gorbachev has now taken in his agreement with Chancellor Helmut Kohl to surrender Soviet rights in Germany as a victorious power of World War II and to grant the reuniting democratic Germany's right to choose its internal development and its international orientation as well.

For the Germans it is a milestone in a year of milestones -- actually, it is barely eight months since the Berlin Wall was breached. The starting point was Mr. Gorbachev's acceptance that Soviet security and welfare would be strengthened, not weakened, if Eastern Europe were let go. Mr. Kohl's no less statesmanlike contribution was not simply to seize the moment, which of course he did, but to seize it in a way that calmed and satisfied both East and West; otherwise, his resoluteness and leadership would have ignited fierce backfires. Mr. Gorbachev, once he started down the German road, put himself under heavy pressure to go the distance. But he would have halted the progress -- and with wide Western approval -- if the Germans had given him reason to. The consensus created in the act of healing the division of Germany and Europe is a tremendous achievement and bodes well.

Important details remain to be worked out in the uniting of the two German states. Soviet-German military talks are to see Soviet troops out of East Germany and to secure German sovereignty and Soviet security in that part of the restored nation. All of Europe, and especially Poland, await a treaty making the German-Polish border inviolate, and there will be a new Soviet-German treaty to regulate bilateral relations. The German army will be reduced and its anti-nuclear obligations reaffirmed. No doubt the Soviet Union expects from the Germans substantial economic aid in various forms; President Bush did what he had to do in acknowledging that the Germans had their own reason to respond.

As Germany is reunited in NATO, a great American mission in Europe is accomplished, and a lesser but still important Alliance mission is assigned: to keep NATO relevant and viable in the new family of international institutions in which the European and Atlantic futures will be worked out. This will produce its own abundant vexations, but they are to be welcomed as the challenges of success.