IT IS odd that so little attention has been paid to the administration plan to put 1,000 or more American combat soldiers into the international peacekeeping force that is due to be placed in Egypt's Sinai desert when Israeli forces complete their evacuation next April. This is very different from the civilian observer group that has watched over the Egyptian-Israeli truce line in the Sinai since the mid-1970s -- a group set up on the expectation that, if trouble started, it would quickly move out of the way. By contrast, the new force would presumably be in the middle of things. It would have to be, to fill its intended deterrent role as plate-glass window that one or the other side would have to break to go to war.

But there is a more important difference between the observer group and the proposed peacekeeping force, and this is the factor that makes it not only necessary but also safe, as these things go, for the United States and the few other likely participating nations to send troops to the Sinai. The observer group was set out between two nations then still formally at war, while the peacekeeping force will be put down between two nations that are formally at peace and that are working in many ways to convert this formality into a living, permanent, irreversible reality. Jimmy Carter committed the United States to such a role, and Ronald Reagan is following through.

The force now being planned reflects a typical Mideast compromise. Israel is fearful, though reluctant to say so too conspicuously, lest President Sadat or a successor changes his mind about peace. Therefore it wants as potent and American a force as possible -- and one not tied to the United Nations, its nemesis. Egypt is nervous lest foreign or domestic critics accuse it of accepting an imperialist-type presence on its sovereign territory. So it wants as slight and international a force as possible -- and one with no connection to the American effort to find places elsewhere to station men for purposes of general American influence in and around the Persian Gulf.

That leaves a not-too-large force with a peculiar and circumscribed mission. Good. Helping two friendly states keep their new peace is worthy in itself. Some, employing a familiar anti-Camp David argument, will contend that the peacekeeping force is a mistake because it will give Israel a degree of security that Israel will use to rebuff demands to compromise with Palestinians. Skillful American diplomacy, however, can turn that around: the more security that Israel's friends bring it on one flank, the more standing they have to expect Israel to meet willing Palestinians halfway on the other.