President Reagan's decision not to ease the ban on the sale of American equipment for a 3,700-mile natural gas pipeline from Siberia to West Germany runs directly counter to an unwritten understanding our European allies had --or thought they had--reached with Reagan at the Versailles summit.

A tip-off to the president's current thinking can be found in the assertion by a White House official that the Soviet Union is "an economic basket case." Thus, "selective steps" to tighten the noose around the Soviets' economy, "can affect their allocations of resources at the margin." Translated, that means that it is wise to deprive the Soviet Union of hard currency it would earn from sales of gas, because that cash would enhance its military capability.

Moreover, the White House argument goes, if Europe becomes dependent on the Russians for gas, that gives Moscow increased leverage over Western Europe's political independence. Presumably, in the presidential mind, any tie that binds West and East has more bad than good in it.

A formal rationale along these lines has been articulated by Thomas C. Reed, a former National Security Council consultant who last week was named a full-time special assistant to Reagan, supporting the hawkish views of Defense Secretary Caspar Weinberger.

Europeans, dismayed by Reagan's latest Cold War move, are convinced not only that it makes good sense to maintain a business relationship with the Soviet Union--because that reduces the chances for a hot war--but that it is wise to minimize the extent to which they depend on the Middle East for energy.

It becomes clear, despite official denials, that Reagan is girding up for a full-scale economic war against the Soviet Union and its satellites, matching a hardening line on arms control that the president displayed in his United Nations speech. Both are decidedly not in accord with the more accommodative vibes the administration gave off in Versailles and at the NATO summit in Bonn.

At Versailles, after fighting for language that would have prohibited government export credit subsidies to Russia and the Soviet bloc, Reagan settled for a milder expression of restraint, suggested by Secretary of State Alexander Haig. It promised "a prudent and diversified economic approach to the U.S.S.R. and Eastern Europe, consistent with our political and security interests."

The American propaganda machine was immediately cranked up to claim a "victory." Europeans were allowed to gain the impression that having got a "first-time" commitment to limit credits to the Soviet bloc--however vague -- Reagan would allow the sale of equipment for the pipeline to go forward. Haig contended that to do anything else would merely hurt American companies, infuriate Western Europe--and would not prevent completion of the pipeline.

Now, a White House official asserts that Reagan never made a deal in Versailles linking the credits limitation with the pipeline approval. "The extension of the sanctions was decided on exclusively because the situation in Poland has not changed, and may have actually worsened," he told me this week. He refused to provide details of what had "worsened."

At Versailles, I didn't hear the situation in Poland linked to the pipeline decision in any conversation or briefing with any administration official. The Reagan team merely said that the president, after the summit, would review what had been accomplished at Versailles on the question of credits, and make a decision on that basis.

Despite Reed's assessment of the Soviet economy, there is a weighty body of opinion that economic sanctions "have had very limited effectiveness in the past." That was the theme of a recent Trilateral Commission report written by three internationally known establishment thinkers-- banker Robert V. Roosa of the United States, Michiya Matsukawa of Japan and Armin Gutowski of West Germany.

In a telephone interview, Roosa said that Reagan's pipeline decision is "absolutely stupid. It not only creates an inflammation in the alliance, but ignores the fact that the Europeans are conscious of the concerns (about dependency on Soviet gas) and have taken steps to meet them."

Roosa ridicules the White House notion that the Soviets can be crippled by denying them hard currency. "So long as we are not going to try to completely quarantine the Soviet Union, we've got to have trade with them, and one of the few ways in which they can get hard currency, so they can pay for the goods they buy, is through the sale of gas. And it makes good sense for Europe to draw on those reserves, so long as they are available."

In Europe, stunned officials are convinced that the Soviet Union will take Reagan's move as a sign of aggression. They believe it will only be counterproductive in terms of Poland. And as for the Atlantic Alliance, the decision raises a more general question of credibility for Reagan, who talked in grand terms at Versailles about the "spirit of partnership" among the summit partners.