MIKHAIL GORBACHEV is to arrive for a Washington summit, on Dec. 7, after all. He and Ronald Reagan will sign a treaty eliminating medium-range missiles -- a treaty that is a political issue in Republican circles but otherwise appears to enjoy broad public support. The two leaders will move on to discuss larger issues of strategic arms plus the other issues of Soviet-American relations.

Why, a week earlier, had Mr. Gorbachev set a summit condition of prior American concessions? There are lots of interesting theories. The important thing is that Mr. Reagan declined to panic and in a short time the Kremlin backed off. For Americans to attempt to play the angles in the Kremlin is foolish. What counts is to calculate the American interest, and then to follow through.

Events are back on the track they were on before the week that wasn't: the track of a confrontation over space defense. Mr. Reagan insists that vigorous American pursuit of his Strategic Defense Initiative is essential to security and peace, and Mr. Gorbachev insists that such pursuit is anathema. Mr. Reagan argues that agreement on a 50 percent cut in strategic warheads -- his priority -- should not be held hostage to space defense. Mr. Gorbachev says logically that offense cannot be fixed until you know what defense it might have to overcome.

On the level of principle it all sounds beyond agreement. Still, the issue may yet be nudged to a more pragmatic level. The Soviet approach is to use the Anti-Ballistic Missile Treaty of 1972 -- the basic common document on defense -- to put agreed limits on testing and development. The Reagan administration approach . . . Well, there are two administration approaches. That's the trouble but that's the brightness too.

One approach recommended inside the administration is to put the foot to the floor on SDI in the hope that the progress thus to be gained will be worth more than any space-defense accord with Moscow can bring. The other -- the right course and the course Congress seems bent on enforcing by its power of the purse anyway -- is to accept certain limits on testing and development in the expectation that nothing important in defense is lost by proceeding at a modified pace and much that is important in reducing the Soviet warhead threat is gained. Mr. Reagan had not made up his mind between these two approaches a week ago, and he still has not made up his mind. How will he go?