It is again a nervous moment in the Middle East. The Reagan administration, having spent a frustrating fortnight trying to rally the European allies to apply tougher economic sanctions against Libya, is notching up the military pressure on its own. It is not that there is evidence that the American government is eager to take a crack at Muammar Qaddafi; some of the familiar cautions that have vexed would-be Libya-bashers are still there to be seen. Rather, the reasoning seems to be that the allies had an opportunity to contribute to American policy by joining collective sanctions, but they decided not to and so the Americans are free to use their own best judgment on how to proceed.

It's probably just as well the Europeans were not consulted on whether they would join the United States on the military front. It's hard to believe that any of them would have agreed, and the refusals would have brought Washington further embarrassment. As it is, the situation is uncomfortable enough. The administration had, in Abu Nidal's Rome and Vienna raids, as good a case for tightening the screws on his patrons in Libya as the uncertainties of real life allow. Its appeals on sanctions were not strident or extreme. But the Europeans are not coming along. While Col. Qaddafi has made his defiance of the United States a pan-Arab cause, the United States is having to deal with him alone.

It is not apparent just what is the purpose of the U.S. naval maneuvers in the Mediterranean is. The official word is that the Sixth Fleet is merely exercising the American right to move about in international waters and airspace -- as though there were any question about it -- and that there is no intent to provoke or bloody Libya. The question is whether Col. Qaddafi, who says the United States is "playing with fire," will respond in the stated American spirit of legality and inoffensiveness. There is the matter of his state of mind. His credibility as a terrorist, moreover, is all too well established.

The Russians have their own large role in this war of nerves. Their past patronage in its various forms has assisted greatly in helping Col. Qaddafi graduate from the nuisance to the menace class. In the current cycle, the Kremlin has moved its (intelligence and fighting) ships closer by, and it has been working on the installation of surface- to-air missiles in Libya. Such acts increase the Soviet commitment to Col. Qaddafi and, in so doing, raise the stakes in any American response to him.

Whether, at this new and higher level of American-Libyan tension, Moscow is restraining the Libyan leader or emboldening him is unclear. But certainly Moscow is not limiting its own policy out of any desire to cultivate better relations with Washington. The opposite may have more truth -- that Moscow wants to show at a moment of superpower negotiations on strategic arms that it is standing solidly behind a threatened regional ally.

The United States, however, cannot afford to let its struggle against terrorism be overwhelmed by its differences with Libya. That gives the Qaddafis of the world too much importance and draws attention from the requirement to go to the political sources of terrorism. One principal source, unquestionably, is the unresolved Palestinian question. The State Department's man for the Middle East, Richard Murphy, has been on the road again, cautiously exploring whether it is possible in coming months to bring Israel and Jordan closer to a negotiation. This quest would be essential even if terrorism were not the concern it is. It marks the leading way that American policy must go.