But US needs backup; Sanctions plan sorely tests allies

President Carter's effort to punish Iran and the Soviet Union through economic sanctions puts America's major allies through a wringer of hard choices.

Unless Japan and WEstern Europe back up the sanctions with restrictive trade policies of their own, US boycott efforts are likely to have little real bite.

Yet, for two reasons, cooperation with Washington on sanctions potentially could hurt Europeans and Japanese more than it does Americans.

* Japan, West Germany, and some other European nations get 10 percent of their oil from Iran, compared with 4.5 percent for the United States before Mr. Carter cut off oil imports from that land.

Iran's oil minister says his country will shut off oil shipments to any other country that follows the US lead in imposing economic sanctions against the Tehran government.

* Japan and West European powers find major markets for their technology and machinery in the Soviet Union, which they are reluctant to lose.

Washington's major industrial allies, with relatively small home markets, export 25 percent or more of their gross national product, compared with less than 10 percent for the US.

All this comes at a time when slow economic growth and rising unemployment impel industrial powers to expand exports wherever they can.

These facts explain why some allied leaders are less than enthusiastic about President Carter's strenuous efforts to enlist their support on the sanctions front.

Some success has been achieved:

* Canada, Australia, and the nine-nation European Community (EC) have agreed not to sell the Soviets additional grain, to make up for the 17 million tons of American corn, wheat, and soybeans now denied to Moscow by Mr. Carter.

Flawing this agreement is the refusal of Argentina, another major grain exporter, to deny extra supplies to the Soviet Union.

Still unclear is how much grain Argentina -- or other smaller nations with stockpiles of wheat and corn -- might make available to official Soviet buyers, who presumably will scour the world for supplies.

* The Japanese government, after a confused and heated diaglogue with Washington, told Japanese oil firms not to buy additional Iranian oil, made available after the US cutoff.

Although Japan's oil storage tanks are brimful, the nation's concern about future supplies is acute. International oil companies, with reduced crude supplies available to them, are cutting their deliveries to Japan.

Experts draw a distinction between the state of military cooperation among Western allies, which is strong, and economic, which is more tentative.

"Militarily," said one top European diplomat in Washington, "the NATO alliance is in good shape, shown by the recent agreement to station a new generation of nuclear weapons on European soil."

Frictions are more evident in the gray area between peace and war, symbolized by the American call for economic sanctions against the Soviet Union and Iran.

Although allied leaders recognize the need for President Carter to respond in some way to the Iranian and Soviet challenges, some experts doubt the efficacy of sanctions to force policy changes in the target states.