A tricky period in allied relations opens this week at the meeting of NATO foreign ministers in Rome. In deference to European opinion, the United States has to appoint a date -- probably this fall -- for resuming talks with Russia on arms control.

In the interim, the Pentagon will be making major weapons decisions that profoundly affect both the allies and the Russians. But no mechanism exists in the Reagan White House for squaring defense choices with foreign policy goals.

Differences between the United States and the allies on dealing with Russia define the danger zone ahead. Most European leaders believe solid gains flowed from the easing of tension, or detente, with Russia achieved by the Nixon administration. In the American view, the Russians used the atmosphere of detente to acquire military superiority and an extension of influence around the world. Thus Washington believes a prime condition for stopping Soviet aggrandizement is a buildup of American and allied military strength.

That conflict in outlook found expression in a "two track" decision made by NATO at the end of 1979 regarding defense of the continent. One track provided for modernization of European nuclear forces by the stationing of 572 new launchers in West Germany, Italy and Britain. The other stipulated simultaneous negotiations with the Russians on reducing all nuclear systems in Europe.

The hostility shown to the Russians by the Reagan administration has stirred up, European fears of a regression from detente to cold war. The Russians have played on these apprehensions by offering to negotiate with the Europeans on arms control or without the Americans. Even allied leaders deeply committed to build up NATO have pressed the Reagan administration to start arms control talks with Moscow.

At the Rome meeting this week, Secretary of State Alexander Haig will agree to begin talks with the Russians about arms control negotiations. He expects to spend most of the summer defining the framework for negotiations in private sessions with the Soviet ambassador, Anatoliy Dobrynin. In the fall, when the foreign ministers convene at the United Nations, Haig would open conversations with Adrei Gromyko about reducing European-based nuclear weapons. In time those negotiations would lead to a renewal of Soviet-American talks on the stalled SALT II treaty governing intercontinental weapons.

The administration hope is that over the summer the Pentagon would use the time, and the pro-defense atmosphere not prevalent in this country, to nail down basic defense decisions. But several of the decisions feed back into the complex of relations among the United States, the allies and the Russians.

The basing of the new MX missile is a case in point. The MX is a very powerful, highly accurate weapon designed to take out Russia's biggest nuclear blockbusters. Basing is important because the MX must be protected against a Soviet first strike. The Carter administration selected a system for moving the weapon on trucks back and forth among a score of different silos on specially built roads in Utah and Nevada.

High costs and local complaints have caused a rethinking, and Secretary of Defense Caspar Weinberger has expressed interest in putting the MX "at sea." But if the United States refused to base the MX on land because of gripes from ranchers in the open spaces of the Southwest, the Europeans would never agree to station nuclear weapons on their far more heavily populated territory. The Russians would find it that much easier to split the United States and the allies.

Another set of defense decisions involves the beefing up of American naval and air forces. Huge spending programs for a new bomber (about $20 billion) and two new carrier groups (about $7.5 billion) are now being talked up in the Pentagon. But the commitment of such sums for sophisticated weapons of only marginal utility now would mortgage the money required for manpower at this time, and for the cruise missiles and submarines that are the weapons of the future. Both the allies and the Russians would doubt the staying power of the American defense effort.

By the standard, the defense choices are hard. A balance has to be struck between maintaining an American buildup and keeping the allies allied. At the same time the United States, as it renews its strength, wants to keep the Russians talking.

Normally the White House would play the decisive role in weighing these choices. But Reagan has no experience in foreign policy or defense. Neither do his three top advisers -- Edwin Meese, James Baker and Michael Deaver. His National Security Council staff lacks the required expertise in the defense area. So if it wants to keep the allies allied and the Russians talking, the White House at the very least might desist from gratuitous shots at the one senior official in the administration well equipped to manage the traversing of the Atlantic danger zone -- Secretary Haig.