Like many Americans, I used to talk glibly about "projecting power into the Persian Gulf" and providing "military assistance" to the Afghan resistance. But that was before I met the top man in the most strategic piece of real estate around those parts.

He is President Zia Ul-haq of Pakistan. I came away from a long interview with him at the presidential palace here in Islamabad persuaded that building American strength in this area is far more difficult than I had imagined. Maybe even impossible.

The central strategic importance of this country is well known to Gen. Zia. He says the Russian invasion of Afghanistan made Pakistan "a front-line state" in the effort to contain Soviet expansion. He also describes Pakistan as the "back door" to the Persian Gulf. "Unless the back door is safe," he says, "the Gulf isn't safe."

With respect to Afghanistan, Zia conceded that "nothing serious" had yet emerged from his effort to promote a political settlement built around withdrawal of Soviet troops. He acknowledged the Afghan resistance needed help, especially in ground-to-air missiles for use against helicopters. He said American assistance should have begun "long ago."

But the necessary "conduit" for such aid had to be Pakistan. The United States and Pakistan had to work out certain "modalities." He admitted that Pakistan needed American assistance -- chiefly in the economic field, but also to modernize the air force and to build roads and bases along the western front with afghanistan. He insisted, however, that far more than western front with afghanistan. He insisted, however, that far more than planes and tanks were involved. Basically, there had to be a feeling of "credibility," or "reliability.

Three conditions, Zia said, posed obstacles to harmony. There was American opposition to Pakistani plans to develop nuclear energy for "peaceful purposes." Zia recalled great difficulty with President Carter on that score. He felt there would be less trouble with the Reagan administration.

The political character of the present Pakistani regime constituted a second obstacle. Zia admitted he did not lead a "representative government." He headed a "military regime." He had started to build democracy at the villiage level and hoped eventually to have national elections. At the present moment, as the hijacking incident illustrated, internal tension was so great that national elections would be "suicidal." Nor could Zia promise a date for a "peaceful return to democratic rule."

Relations with India raised the third and most important obstacle. Zia said that India was a "great power" with 650 million people and the Pakistan, a "small country" with only 80 million people, was not "in competition." But he insisted that American military assistance be given to Pakistan "on the merits" and not as a function of American relations with India. He said that he had tried to improve relations with India, without much success, and he doubted the United States could do much to promote harmony.

There was something fishy, however, about all his examples. At one point he said Prime Minister Indira Gandhi was "allergic" to American aid to Pakistan. I remarked that Gandhi would not like the term "allergic." Zia agreed, but kept on using the term. I concluded that what he really sought from the United States with respect to India was the right to make with impunity rude gestures, like thumbing his nose, or sticking out his tongue.

As to the Persian Gulf, Zia acknowledged that all the Gulf states were weak. He said that they depended on outside help for manpower, equipment and technical know-how. He asserted the Russians could easily move into the northern parts of Iran, or apply pressure at the Strait of Hormuz, the choke point through which most of the oil passes.

Still, he insisted over and over again that the security of the Persian Gulf was essentially a local affair. "basically," he said, "the security of the Gulf is the concern of the Gulf states."

I asked him about the possibility of Pakistani troops helping to strengthen the monarchy in Saudi Arabia. He said help would have to be limited to such matters as building roads. He was cool to the notion of an American force on land or sea. He claimed American bases in the area would imply "a tacit agreement with the Soviet Union as to areas of influence -- a second Yalta." a

I suggested to Zia that "a front-line state" had to have access to some heavy force. He demurred. He said that for Pakistan the "first pillar of policy" was "association with the Moslem world." The "second pillar" was "our link with China." Those two relationship could best be "developed through the agency of the nonaligned countries and the United Nations."

"How about the United States?" I asked, in a how-many-divisions-does-the-nonaligned-world-have tone of voice. He said: "It is not humanly or militarily posible for the United States, from 10,000 miles away, to support Pakistan. . . ."

Maybe President Zia is playing hard to get. Maybe he is trying, for local political consumption, to show that he cannot be bought. But I don't think so. My sense is that he believes the United States is an unreliable ally, and that he prefers to put his trust in the United Nations and the Third World. If so, there is not much the United States can do. One possibility is to divide up the local forces -- to try to achieve, in other words, a balance of weakness.