ONCE AGAIN a Pakistani has been arrested in this country for trying to smuggle out materials that could be used to build nuclear weapons. This time it was steel of a certain highly special type used in the uranium enrichment process. Work is proceeding steadily in Pakistan, and it is pressing the United States toward an unpleasant decision. American law forbids economic or military aid to countries that are building nuclear bombs. But Pakistan provides the major supply routes and staging areas for the guerrilla resistance to the Soviet occupation of Afghanistan, and for that reason the United States currently gives it very substantial aid. Pakistan is forcing the United States to decide between its interest in sustaining the Afghan resistance and its interest in preventing the spread of nuclear weapons. Put in those stark terms, the greater necessity is to deflect the nuclear rivalry -- and with it, the possibility of nuclear war -- in South Asia.

Pakistan has pursued nuclear weapons with obsessive energy ever since India exploded one in 1974. Since legal sources of the necessary equipment have been foreclosed by a worldwide embargo, Pakistan has resorted to systematic theft and smuggling to get the technology. There have been many recent cases in which police and customs agents here and in Europe have intercepted illicit shipments; it's clear that there have also been many shipments that reached Pakistan.

The recent pattern of these cases is ominous. Two years ago a German company evaded the embargo and sold Pakistan a ton of the special steel used in enrichment equipment. The Pakistani arrested this week in Philadelphia was attempting to obtain 25 tons of the same steel. In April West German authorities raided the headquarters of a Cologne firm that makes components for Urenco, a consortium that enriches uranium for several countries' power reactors. It appears that the Cologne firm had been supplying both blueprints and equipment to Pakistan. Since Pakistan was already capable of producing enough enriched uranium to build perhaps one bomb a year, the latest cases suggest that it is now trying to expand production.

In the 1970s the United States cut off aid to Pakistan because of its reckless pursuit of nuclear weapons. But in 1979, when the Soviets invaded Afghanistan, this country immediately reinstated the aid. Now the Pakistanis seem to believe that because of Afghanistan the United States will never enforce its nuclear control law and withdraw aid again. Are they right