Private warnings to presidential aides last month that the neutralist regime in Afghanistan was "ripe like a red apple" for a pro-Soviet communist takeover met official silence here, a non-response highlighting the administration's dangerous inertia in meeting the current Soviet worldwide offensive.

That warning was quietly passed to top Carter foreign-policy officials three weeks ago. The source was a trusted, Teheran-based emissary of the shah of Iran.

Even if the United States were not tied into a strait jacket imposed by Congress as a result of post-Vietnam politics, it would have been impossible for President Carter - or any other president - to prevent the pro-Soviet takeover or influence its timing. But the warning from the shah, and similar warnings from Pakistan, had no effect at all on the Carter administration.

Murmuring soft sympathies for the shah, administration officials lost a particularly timely occasion to notify Moscow sharply that Soviet support for a communist takeover in previously neutral Afghanistan (which borders the Soviet Union) would have disquieting repercussions in Washington.

That ostrich-like, see-no-evil posture of Carter foreign-policy planners fed doubts among the U.S. allies - doubts now reaching agonizing proportions - the Soviet offensive in Africa and Asia is incapable of engaging Jimmy Carter's interest. Instead, those allies believe, Carter's current policy has a single aim: do nothing that might offend Moscow and put at risk the planned summer summit meeting with Soviet President Leonid Brezhnev to sign a new strategic arms limitation agreement.

But that may be too benign a view, a conclusion that is shared by observant diplomats here and high defense officials. "I'm not sure that Jimmy Carter has a global perspective," one told us. "He tends to see things that happen abroad as isolated incidents, not linked together."

An even more melancholy view, widely held by experts here, is this: Presidential worry that a tough warning to Moscow not to push the United States too far might well require an equally tough U.S. follow-up, one for which the Carter administration is unprepared.

But, in fact, installation of the pro-Soviet regime in faraway Afghanistan sets the stage for a series of new communist probes that will confront Jimmy Carter with ever more difficult choices. Those future events, spawned in the bloody Kabul takeover, are what caused the shah to sound his alarm three weeks ago.

It is highly probable that the new rulers in Kabul soon will exploit ancient territorial disputes about tribal lands adjoining both Pakistan and Iran. Exploitation of the most important of those disputes would establish an independent state in what is known as Baluchistan, a slice of prime strategic territory along Pakistan's western border with Iran that runs to the Indian Ocean.

Access to the Indian Ocean has always been a Russian dream. In the glory days of the British empire, wars were repeatedly fought to prevent Russian penetration south through what was British India to warm-water ports on the Indian Ocean.

No immediate move like that is expected from the new communist regime in Kabul, but agitating those ancient tribal rivalries across its borders would foster Afghan nationalism. That is a proven method of building political support for any new regime, and is particularly useful for a communist regime trying to consolidate its power in orthodox Moslem state of Afghanistan.

But even without such provocative politics, installation of the pro-Soviet government has already compelled Iran to reinforce its eastern frontiers with both Afghanistan and Pakistan. The takeover is a second giant step toward what the shah has always warned against: encirclement of the oil-rich Persian Gulf region including Iran and Saudi Arabia, by Moscow and its satellites. The first was Soviet penetration of the Horn of Africa.

For an administration seemingly so preoccupied with SALT, the question of when or how to come to grips with disconcerting issues like faraway Afghanistan and the Horn of Africa goes to the backburner. Indeed. when Air Force Secretary John C. Stetson said on April 21 that the United States had a "tacit obligation to back up" Iran in the event of a hostile Soviet move, he was publicly rebuked by the State Department for using overly strong language.

That was just before Moscow laid claim to communist power in Kabul. Since then, there has been no visible change in Carter's policy. Quite the opposite: The change has been one more demonstration of Soviet disdain for U.S. will and one notch tighter in the psychological fear campaign against U.S. allies.