The week of damage control. After squabbling over Libya, US and Europe both give a little

This was damage control week for the NATO allies. Having fallen out with each other the previous week over the United States air strike on Libya, the allies in Europe were this week busily expelling suspect Libyans and tightening police and airport security against any potential troublemakers.

In return, the US government took a major step which greatly pleases and relieves the European allies and other friends, including the Japanese. The US decided not to cancel the second, unratified Strategic Arms Limitation Treaty (SALT II). Instead, the US will scrap two obsolete Poseidon submarines in order to keep under the SALT II limit on long-range nuclear launchers as the USS Nevada, latest of the new Trident strategic submarines, comes into service later this year.

The decision to continue to honor the limits in SALT II will sweeten the atmosphere for the big summit meeting of the industrial democracies, which begins in Tokyo on May 4 and reopens the path to the next Reagan-Gorbachev summit, which was temporarily blocked by the fallout from US bombing of Libya.

Soviet leader Mikhail Gorbachev had virtually said that he would not come to Washington without assurance that useful negotiations were in sight. Breaching the SALT II limits would have been assumed in Moscow, and in allied capitals as well, as meaning that the US was discarding the whole arms control process.

The decision at the White House on SALT II was crucial. Ever since the Reagan administration took office in Washington, the ``hawk wing,'' led by Caspar Weinberger and Richard Perle at the Department of Defense, has crusaded passionately for the US to break away from all arms control agreements and go ahead building any weapons it wants to build.

A moment of decision on this highly controversial question arrived at the White House over the question of what to do when the Nevada is commissioned. She is being fitted out at New London, Conn., and will begin her sea trials in May. She is to be commissioned in October. She carries 24 launchers for triple-headed nuclear missiles.

SALT II sets a limit of 1,200 on the number of multi-warhead missile launchers that each country may have. The US now stands at 1,178. The 24 on Nevada would put the US over the limit by two. Each Poseidon submarine carries 16 launchers.

The Weinberger-Perle faction urged that if the two Poseidon submarines were decommissioned to make way for the new 24 launchers on the Nevada, they should be mothballed, rather than scrapped. That might have been taken by the Soviets as a violation of the treaty limits, since they could be recommissioned. The President agreed to scrapping instead of mothballing.

This is not a final defeat for the Weinberger-Perle faction. Every time a new Trident submarine hoists her commission pennant, something else will have to be scrapped to keep inside the limit. In December, there will be more B-52 bombers converted to carry air-launched cruise missiles fitted with nuclear warheads.

The hawk faction will of course argue for a breaching of the treaty limits every time the issue comes up. But the issue over Nevada's launchers was the last best chance they had for treaty breaching before President Reagan meets Mr. Gorbachev on the next scheduled summit. The present expectation is that this will occur either in July or shortly after election day in November.

The fear in the hawk camp is that Mr. Reagan might, at that next summit, take a long step toward a new arms control agreement.

The surest way to head off such an event would have been to win over the President, at the decisive April 16 meeting on the Nevada's 24 launchers. The pro-arms control faction, led by Secretary of State George Shultz and Paul Nitze, the President's chief arms control adviser, won that round.

The allies in Europe and Japan, are all strongly in favor of honoring the SALT II limits and using them as a foundation for new arms control agreements. This is particularly true for Britain.

A rumor circulated this week that British Prime Minister Margaret Thatcher made survival of the SALT II limits a condition for allowing the use of British air bases for the strike on Libya. While untrue, it accurately expressed the importance of the SALT II decision to Mrs. Thatcher and Britain.

One important fact lay behind the decision to scrap the two Poseidons and observe the treaty limits. The Soviets have assembly lines for long-range nuclear weapons in operation. The US does not. The US is working toward the new MX missile. It is still in the research-and-development phase. There is no assembly line ready to run.

The Soviets have been developing new versions of their strategic weapons. Their assembly lines are running. They could go into high-speed production of more weapons in a matter of days.

The military leaders at the Pentagon want the new MX missiles more than they want freedom from SALT II limits. The Joint Chiefs of Staff have said that the decision on SALT II observance is political, not military. Mr. Reagan campaigned against SALT II in 1980. It no doubt was difficult for him to accept the idea that keeping the SALT II limits can be to US advantage.