The U.S. proposed imposition of Western Economic and diplomatic sanctions against Iran is likely now to produce a series of repercussions the West previously had sought to avoid -- closer Iranian ties with the Soviet Union, and the creation of new Third World allies of Iran.

Iran has already been fairly successful in making preparations to withstand an international siege by strengthening its non-Western trade relationship and using Soviet land routes to carry increasing amounts of imported goods into the country, according to reports from Tehran, Moscow, New Delhi, Ankara and London.

Iran is like a triangle, with oil, its only significant export, flowing down the long funnel of the Persian Gulf. Many of its imports -- food, raw materials, spare parts and medical supplies -- flow up the same route.

But this is mainly because the sea and the new ports developed under the deposed shah represent the most convenient and cheapest route. There are plenty of alternatives, if Iran wants to use them, and there is plenty of evidence that Ayatollah Ruhollah Khomeini is ready to resist any new turns of the Western screw.

The northern edge of the Iranian triangle is the Soviet border. Last January, when the United Nations security Council voted 10 to 2 in favor of imposing sanctions on Iran to secure the release of U.S. hostages, it was Moscow's veto that stopped President Carter in his tracks. Now, the Soviets are stating publicly that they will make land routes available in the event of a blockade.

The main link here is the direct rail line from Tehran to Moscow, crossing the Iranian frontier at Jolfa. This small town has been heavily developed, with substantial warehouse facilities (as there are farther south, at the busy industrial center of Tabriz), and recently more than 300 freight cars a day have been crossing from the Soviet side.

Roughly 75 percent of the cargo originates not in the Soviet Union, but in Japan and Western Europe. The service could be easily switched to bring goods from Taiwan, South Korea and Eastern Europe, should the West abandon the Iranian market.

That market is by no means as depressed as it was last year, after the fall of the shah. After dropping by nearly two-thirds, trade is picking up briskly. Britain expects to sell nearly $880 million worth of goods to Iran this year, unless political obstacles intervene, and most other suppliers report similar, or even better, prospects.

Important road links through Turkey, notoriously clogged and requiring lavish bribes during the shah's time, have also been reactivated. Ankara is under heavy U.S. pressure to fall in line with Western policies, but Prime minister Suleyman Demirel is deeply dependent on Moslem support, and neither his Cabinet colleagues nor the opposition would tolerate serious moves against Iran.

To capitalize on this, Tehran has offered cash-strapped Turkey 1.6 million tons of oil on easy terms, agreed to lift all transit charges and opened the frontier to local trade, meaning food for northwest Iran and ample diesel oil for eastern Turkey.

In addition, Iran has held intensive talks with all the Eastern European countries about alternative raw material supplies; Thailand and Pakistan are being lined up as rice suppliers. Pakistan, too, is accessible, by road and, to some extent, rail, and President Mohammde Zia ul-Haq has gone out of his way to express sympathy for his Islamic neighbors.

With one of the world's busiest smuggling coasts along the Gulf of Oman -- outside the limit of a American naval blockade -- Iran is not short on entry doors.

Iran also has signed new alliances with Algeria and Libya, and a variety of tempting offers have come from as far afield as India and bangladesh, in case the west Europeans pull out.

India, which is considered a major source of goods, has been asked for rapid deliveries of eggs, meat, edible oil, rice, soap and assorted consumer goods. Big gainers, again, could also include the Soviet Bloc. Massey-Ferguson's profitable Iranian tractor business has been eyed greedily by Romania for years and several Eastern European car plants, notably Skoda and Lada, believe they could pick up a slice of anything British manufacturers care to abandon.

Iran is not going hat-in-hand to the Eastern Bloc, however. While Iran's toughness on oil prices has been driving Western boardrooms to fury, the Soviets have received equally cavalier treatment. A Soviet negotiating team left in a huff recently after Iran demanded a fivefold cost increase for the natural gas it pipes in vast quantities to the Soviet Union.

Now, the Soviets are back at the negotiating table, with their tails between their legs, to work out new deals for this and other trade arrangements. Obviously, one effect of Western sanctions could be to help them win better terms.

Not likely to be short of food or other supplies, neither is Iran short of money. As domestic purchasing has dramatically decreased, Iran's foreign currency reserves, even allowing for the freezing of its American assets, are estimated at $7 billion.

This is used to buy wheat and meat (mostly from Australia and New Zealand); barley; sugar (from Latin America and central Europe); a variety of industrial raw materials and spare parts for the part of Iranian industry that is still functioning.

This last is Iran's only area of real vulnerability. The oil industry, which relies on highly specialized parts made only in the United States, is suffering badly. It is only the low level of output and extensive cannibalization from unused plants that keeps things going. Computers, too, although there is a locally owned IBM subsidiary, are an area of growing difficulty.

But $7 billion and the possession of even moderate oil and gas supplies give Iran substantial arm-twisting power. West Germany, with the biggest slice of Iran's import business, and Japan, with its showpiece $3 billion petrochemical plant at Bandar Khomeini, close to completion, still rely on Iranian fields at Abadan for 12 percent of their oil needs.

Ayatollah Ruhollah Khoneini has made it clear that taking part in further economic sanctions will mean an immediate cutoff for those nations' energy supplies. Most other Western nations have sharply cut back on their Iranian oil dependence -- in the case of Demark, to zero.

But even Demark, with its big agricultural exports, is not indifferent to the prospects of loss. Although in a pinch the Western partners place priority on its friendship with the United States, it will be with reluctance and pain that they accept any financial sacrifices.

Their moves toward sanctions suggest that the Europeans are aware that Carter's patience is not inexhaustible, and that if pushed too far, he will take military action to solve the hostage crisis.