"The United States has an important leadership role," Ronald Reagan said in Step Two of his nine-step outline of a comprehensive foreign policy on television the other night, "and this role can only be effective if our alliances are cemented by unity of purpose and mutual respect."

Right on! No argument with Jimmy Carter there.

But then the Republican candidate moved on from Step Two (relations with friends) to Step Six (strategic arms control). And right away he not only reconfirmed yet another nice, neat and fundamental difference between him and the president on foreign policy. He also raised a big question about Step Two -- about just how much "unity of purpose" Ronald Reagan as president would be able to establish with this country's European allies.

Reagan would tear up the SALT II arms control agreement initiated by his two immediate Republican presidential predecessors and ultimately negotiated and completed with refinements and some revision by President Carter. Reagan thinks it would merely slow the arms race while giving important advantages to the Russians. So he would plunge instantly as president into negotiations on a SALT III agreement with a goal of actually reversing the nuclear arms race by achieving mutual reductions in nuclear arsenals.

Carter would simply proceed as rapidly as possible to push for Senate ratification of SALT II, on the theory that 1) that's as far as the Soviets are now prepared to go and 2) SALT II is a logical and essential intermediate move before attempting the actual cutbacks that Reagan would have as his initial objective.

That's their difference -- but not without a certain irony: what Reagan would make his first arms control order of business Jimmy Carter actually made his, two months into his first term in 1977. His failure was, well, embarrassing. The Soviets were not just resistant. Their rejection was resounding.

While is was said at the time that the Carter initiative was too sudden, or badly handled, it had significant domestic support, most particularly from Sen. Henry M. Jackson (D-Wash.), who is generally not thought of as softheaded about the Soviets. In any case, almost all of the experts I have talked to agree that a similar effort to bypass SALT II in favor of the bolder SALT III arms reduction approach would find even less favor with the Soviets today.

Especially if accompanied by the massive arms buildup that Reagan proposes as a means of improving the American bargaining position, the scrapping of SALT II in favor of a leap directly to SALT III would have the effect, these experts argue, of simply postponing indefinitely and U.S.-Soviet negotiations on nuclear arms control.

And this is precisely where Reagan's Step Six (strategic arms control) comes sharply into collision with Step Two -- relations with allies "cemented by unity of purpose." In truth, SALT II constitutes a very large part of what cement there is between the purposes of the United States and the Europeans when it comes to dealing with the Soviets.

The Europeans may be wrong about SALT II, but they strongly support it. They have sent emissaries from North Atlantic Treaty Organization headquarters to lobby the Senate for its ratification. The West Germans, who are central to NATO defenses, feel perhaps most strongly. But most Europeans see SALT II not just as a useful part of an essential process of winding down the arms race but as the true test of that which they most cherish: detente.

That the Europeans have little stomach for East-West confrontation and much enthusiasm for increased East-West accommodation (trade, cultural contacts and all the rest) was evident in their response to Afghanistan. It is all the more so in their almost furtive cooperation with American efforts to establish an impressive military presence in the Persian Gulf area.

Much has been made of the powerful "allied" armada now assembling in the Indian Ocean, presumably on call to clear the vital Strait of Hormuz, if need be. But the way the French (or the Australians) describe their contribution to it, it sounds as if they just happened to be in the neighborhood and decided to drop in. The West Germans insist their constitution forbids them to venture so far from home for an allied show of solidarity.

The Europeans cannot deny their extraordinary stake in the free flow of oil. But they are nonetheless profoundly reluctant to identify themselves with a great show of force -- West against East -- in that part of the world. Instead, they offer little more than quiet diplomacy and perhaps some increased defense effort within NATO to free up the United States' role as the allies' designated pinch hitter in the Gulf.

Carter's juggling of those conflicting allied needs and inhibitions may leave much to be desired. But he does recognize that they do exist. Reagan has yet to explain how he would reconcile his insistence on scrapping SALT II with the allied "unity of purpose" he says is so essential to a leadership role for the United States.