Mr. REAGAN, who leaves for Europe today, smoothed his way with his Memorial Day pledge on SALT II. This is, of course, the "fatally flawed" treaty whose ratification he labored mightily to block. Now he finds it useful, if only to calm down critics.

It is, nonetheless, good to have a presidential reaffirmation of SALT II. The reason is not simply that it will reassure the allies. It is also that Mr. Reagan's START proposals will require major policy changes by a Soviet leadership in the throes of a succession and will take years to negotiate as well. In the interim, it will be essential to have as many useful understandings and agreements in place as possible between Moscow and Washington. The question is whether the president has done as much for himself in this connection as he should.

He has matched Leonid Brezhnev's pledge to respect SALT II if the other does, too. That presumably means the two countries will not build weapons that the treaty prevents them from building. But, by not ratifying, Mr. Reagan evidently loses the provision requiring the Soviets--but not the Americans--to reduce by 10 percent their missile launchers and heavy bombers. He apparently also loses use of the treaty's verification panel to check out questions about Soviet performance.

By bowing to "existing strategic arms agreements"--a foggy formulation--Mr. Reagan evidently means to avoid being pinned down on whether he will continue respecting the 1972 ABM treaty limiting defensive missiles. The significance of this is that "Dense Pack," the latest proposal for deploying the MX missile, entails a defensive system that violates it. From that formulation, furthermore, the president excludes two other agreements, the threshold test ban and peaceful nuclear explosion treaties, signed in the mid-1970s but never ratified. His arms control chief, who came aboard favoring early ratification, reports that new resistance-- in the Pentagon--has developed.

Take these unfulfilled arms control agreements, throw in assorted unratified human rights treaties and the unsigned law of the sea, and you have a country with a richly earned reputation as an unreliable negotiating partner. Other countries may have their own reasons for staying at the table, but the recent American record has got to make prospective partners wonder about the wisdom of investing the time, energy and political capital it takes to deal with the United States. The only consolation is that it is hard to imagine any past or future treaty that Mr. Reagan would send up to the Senate that would not be approved in a snap.