Washington — America's European allies fear ''de-coupling'' in the democracies' common confrontation with Soviet Russia.
-- The Joint Economic Committee (JEC) of Congress here heard sharp criticism from European spokesmen of United States sanctions against those who aid building the 3,500-mile Soviet natural gas pipeline to Europe.
-- A Senate appropriations committee voted 12 to 1 to reduce America's current force of 330,000 soldiers in Europe, partly as an economy measure, partly to warn European allies of America's problems. Congress may reject the committee's lead.
-- Defense Secretary Caspar W. Weinberger said he did not dismiss charges that the Soviets are using slave labor to build the pipeline, an issue in the de-coupling controversy.
-- In Bonn, West Germany, former Chancellor Willy Brandt, the Social Democratic leader, made the US pipeline sanctions an issue, charging political opponents were soft in resisting American pressure.
These and other developments are tied in with the administration's sanctions against US firms that aid the Soviet pipeline direct, or cooperate with West European or Japanese firms in the same undertaking.
Andrew Knight, editor of the London Economist, told the JEC that Washington's action was like ''America's president shooting his closest allies in the foot.''
Mr. Knight discovered ''outrage'' on both sides of the dispute. ''The stakes this time seem higher only because America seems to lay down economic law from a less overwhelming pedestal,'' he said. Europe is stronger than it was, he argued.
''Slowly and often imperceptibly, Europe's and America's views of trade and credit with Russia and eastern Europe have been growing apart,'' he said. He urged Mr. Reagan to use ''the quiet diplomacy of Secretary (of State George) Shultz'' to bring European partners to the table: ''There is a lot of talk about and it should have been talked about long ago.''
Thierry de Montbrial, of the French Institute of International Relations, urged the US to refrain from ''unilateral action and unnecessary harsh rhetoric, '' which he charged would encourage ''de-coupling.''
Andre Fontaine, editor of the French daily newspaper Le Monde, testified that ''no European government is ready to yield to American pressure'' because they are afraid ''to lose the coming elections.'' He argued that it is ''a problem of dignity and sovereignty.''
The distinguished trio of European witnesses assembled here by the Congressional Joint Committee to discuss the Soviet pipeline avoided personalities, but sharply criticized the Reagan policy of sanctions.
While these examples of European reaction were being given to Congress in a public session, other developments involved phases of East-West relations.
Charges in Congress that NATO allies should pay more for defense came in the 12-to-1 vote in a Senate appropriations subcommittee. The issue hasn't been raised significantly in Congress since 1971 when Mike Mansfield (D), then senator from Montana, sought to cut the US force in Europe in half. His proposal lost 61 to 36. The new test indicates shifting patterns. The lone dissenter was Sen. John C. Stennis (D) of Mississippi, ranking minority member, who wanted to uphold the president's hand and NATO allies. Conservative Sen. Jake Garn (R) of Utah, by contrast, said he wanted to send the Europeans a ''message'' that American patience is being tried. The subcommittee recommended a freeze in US troop strength at 1980's level of 330,705. The Pentagon is asking for more troops.
Another argument against the Soviet gas pipeline is the charge that it is employing slave labor. Secretary Weinberger said here, ''The evidence is not conclusive - I hasten to say that. But the available evidence is profoundly troubling and some have found it very persuasive.''