Three months ago Portugal decided to sell to Colombia 30 torpedo launchers on some Sapnish gunboats it didn't need anymore. Because the British manufacturer of the torpedo launchers had utilized U.S. supplied technology, Portugal requested and received permission from the United States to make the sale.

About the same time, the shah of Iran decided to send some of this U.S. made weapons to Somali as a counterweight to Soviet and Cuban backing for Ethiopia.

Washington, however, had decided to stay out of the Somali-Ethiopian war. According to a State Department official, the shipment of arms was canceled due to Washington's objection, after it had been loaded aboard an Iranian cargo plane.

In mid-march, Israel used U.S. supplied warplanes, missiles, artillery and cluster bombs - without U.S. permission - in invasion of southern Lebanon. The State Department began an immediate investigation, an in early April Secretary of State Cyrus R. Vance announced that Irael's action "may have" violated U.S. law. Last week the National Association of Arab Americans, a pro-Arab lobby group, filed suit in federal court to stop all arms shipments to Israel because of the possible violation.

These incidents and dozens more are examples of the potentially long reach of this country's Arms Export Control Law. As the world' leading arms exporter, the United States is also leading exponent of a continuing right to limit how the weapons it sells are used, and to whom they can be resold.

These features of the arms laws are, in effect, a U.S. string on the planes, tanks, bombs and other weapons it supplies to other nations.

The effectiveness and consequences of these controls are major elements in several foreign policy controversies before Congress. Concerning the proposed sale of U.S. warplanes to Saudi Arabia and Egypt, lawmakers are debating whether U.S. law would be effective in preventing the use of the jets to attack Israel or their retransfer to another Arab nation which might do so.

In a separate controversy, lawmakers are debating whether Turkey's 1974 invasion and occupation of Cyprus with U.S. weapons - without U.S. permission - should continue to be punished by an embargo on arms sales to the Turks.

Since the United States in 1947 undertook its post-World War II role as an international supplier of weapons and military equipment, it has asserted that its arms can be used only for the purposes for which they are supplied usually self-defense and internal security. In view of the historical U.S. revulsion against "merchants of death," some limitations may have been necessary for a weapons-supplier role in peacetime to be politically viable.

Several developments of recent years have made the limitations of growing importance. One was the spread of U.S. weapons to almost every corner of the globe.

According to Pentagon data, U.S. weapons were supplied in 1960-1977 to 161 countries or political entities, from small islands to major military powers. Foreign military sales agreements in that period came to $71 billion.

The rise of congressional concern, due to the growing consciousness of U.S. arms sales overseas, resulted in increasingly specific provisions of law. And in the past several years, for the first time, Congress carved out for itself a major role by requiring detailed reports of proposed arms sales and retransfers as well as reports of any suspected violation by foreign nations of the laws on end use and retransfer. Congress also gave itself the right to stop arms sales or transfers it did not like.

All this, along with President Carter's expressed desire to restrain and exert greater control over arms sales abroad, has raised foreign-policy questions of great importance.

From the point of view of foreign goverments, the U.S. string on arms can be a practical limitation on its foreign policy - or the source of political trouble with United States.

For example, Israel's unofficial reaction to the assertion that it misused U.S. arms by invading Lebanon was angry. Former defense minister Shimon Peres, on a Washington visit last month, remarked that weapons which can't be used don't qualify as weapons at all.

Israel's use in Lebanon of cluster bombs - the murderous projectiles which send out thousands of steel shards, like giant grenades - has been a particularly sensitive problem. In late 1976, after congressional complaints, Israel agreed to use these weapons only against fortified targets and only if attacked by more than one country. This limitation was disregarded in southern Lebanon, but in an exchange of notes last month Isreal agreed to abide by the restrictions in the future.

The State Department, in a report to Congress last June, noted that in view of the increasing quantity of U.S. supplied weapons around the world and with militarily advanced countries tempted to sell off old U.S. weapons which they have replaced with new ones, "the risk of unauthorized transfers increases substantially."

The report noted that NATO allies have complained about the strict U.S. controls and that punishment for violations may well be regarded as "punitive" or "an infringement of sovereignty" by nations which paid for their U.S. weapons.

From August 1976 until last month the State Department submitted to Congress only 25 requests by foreign countries to retransfer U.S. arms to other nations. During the same period Congress received only four reports of resales without permission in violation of the law.

These were Israel's sale of Gabriel missiles with U.S. components on patrol boats to South Africa, Israel's initial sale of warplanes with U.S. jet engines to Honduras (three later sales were made with U.S. permission), Yugoslavia's sale of old U.S. tanks to Ethiopia and a recent violation which is classified.

In view of the many thousands of U.S. weapons and component parts around the world, the small number of reports to Congress raises questions about the effectiveness of the restrictions on resale.

From the point of view of policy-makers in the executive branch, the official and public nature of U.S. control over the retransfer and end use of American weapons is a mixed blessing.

Open U.S. military intervention is nearly out of the question in the post Vietnam era, and covert Central Inteligence Agency operations are sharply restricted.

The supply of American arms, either directly or indirectly, is among the last remaining tools for dealing with small-scale conflicts and brushfire wars in remote areas - but the requirement for congressional approval makes their use more difficult.

In an earlier era, the United States could encourage Iran to put down an insurgency in Oman on the Arabian Sea Congress. But the shipment of U.S. arms from Iran to Somalia early this year would have required formal agreement and a report. This may have been among the reasons it was dissaproved.