By abruptly resigning as Soviet Foreign Minister, Eduard Shevardnadze has in effect splashed some shockingly cold water in the world's face.

It is not just the fact but the manner of Shevardnadze's departure that is jolting. Warning that the Soviet Union is crashing toward dictatorship, Shevardnadze gave voice to some widely shared but seldom expressed fears.In essence, the fear is that if the Soviet Union does not get a grip on its slipping economy and the growing demands of its republics for independence, the new era of greater personal freedom and democracy could soon end.

In its place could quickly come a return to the bad old days of rule by the iron fist, either under President Mikhail Gorbachev himself or under some harsh successor.

If that happens, it would be hard to maintain the momentum of recent East-West cooperation. The demise of that process, in turn, would limit or even eliminate the prospect of further arms control agreements and cast doubt on the reliability of pacts already worked out.

But even if the worst never happens, the resignation of Shevardnadze deprives Gorbachev of a close, effective supporter at a time when he needs all the friends he can get. It also demonstrates the increasing strength of resistance to Gorbachev's reforms on the part of the bureaucracy, hard-line Communist party veterans and the military. And it calls attention to the key role played by Shevardnadze in efforts to reform the Soviet Union.

Though Shevardnadze was often seen simply as an executor of Gorbachev policies, it was clear to insiders in the world of diplomacy that the Soviet foreign minister had his own ideas and often exerted a great influence on Gorbachev. What's more, Shevardnadze's flexibility and non-ideological approach to problems eased the negotiation of major East-West arms agreements.

Under Shevardnadze, the Soviet Union pulled out of Afghanistan and began negotiating huge cuts in nuclear weapons stockpiles. It also let East European satellites oust hard-line Communist leaders and reversed antagonistic relations with most Cold War enemies.

Powerful leaders, however, often attract powerful enemies. So it was with Shevardnadze. For ending Soviet domination in Eastern Europe, he earned the enduring enmity of the military. Old-line ideologues reviled him for advancing the idea that class interest should play no role in foreign policy, meaning an end to foreign intervention.

Meanwhile, the shock waves that world capitals are feeling from Shevardnadze's resignation are bound to jolt the Kremlin, too. It won't be easy for Gorbachev to pick a new foreign minister without antagonizing either his supporters or his critics. If Gorbachev tries to placate the critics by resorting to force and repression, he will lose both the foreign and domestic support won by his reforms. At the same time, there are limits to how much more personal power Gorbachev can reasonably expect to get when he has so little to show for previous increases in his authority.

What a muddle! It's a muddle that is perhaps best explained by the fact that the Soviet Union, under Gorbachev and Shevardnadze, has been undergoing a revolution. Even bloodless revolutions are never easy to control and don't always succeed even when undertaken for the most acceptable reasons.

At this point, almost anything could happen in the Soviet Union. About all that can be said for certain is that with the departure of Eduard Shevardnadze, the future of the nation has for the time being become darker and harder to read.