Shortly after the second seizure of the U.S. Embassy in Tehran on Nov. 4, Gen. Alexander Haig, in private talks with politicans and businessmen, accused the Carter administration of assigning his NATO deputy to hasten the shah's fall as Iran's ruler a year ago.

That was given by Haig as a major reason for his resignation in July as NATO supreme commander and his retiirement from the Army. Never before, has undermining the shah been listed as a purpose of the shadowy mission to Tehran early last January by Air Force Maj. Gen. Roberty E. Huyser, Haig's deputy.

Haig, who is eyeing a long-shot bid for the Republican presidential nomination, has not gone public with his sensational charge. When asked by Washington newsmen over breakfast Nov. 21 why he had left NATO and the Army, Haig never mentioned the Huyser mission. Nevertheless, his private chats have fired the opening round of a battle with profound political implications: "Who lost Iran?"

Whether or not Haig's interpretation of President Carter's motives is accepted, he is supplying previously unknown information about upper-lever Washington intrigue as the shah toppled. Here began the administration's policy of making common cause with revolutionary impluses at the expense of old allies.

The policy took effect with a transatlantic telephone call early last Janauary from Gen. David Jones, chariman of the Joint Chiefs of Staff, to Haig at NATO headquarters in Mons, Belgium. haig learned for the first time that the Cater administration planned to dispatch Huyser, who had exceptional contacts with the Iranina military and the royal palace, to Tehran.

Huyser's mission, as explained by Jones to Haig, was "to keep the Iranian military united and effective." That meant urging the Iranian generals not to attempt to coup against the shaky new civilian regime of Shahpour Bakhtiar -- the description of the mission given the press.

Haig regarded this as a smoke screen. Secretary of State Cyrus Vance, in ascendance over national security adviser Zbiigniew Brzezinski, wanted the shah quickly removed from power. To Haig, the Huyser mission promoted this plan. He informed Jones on the telephone that night that he did not want himself, his deputy or the U.S. military involved in what he viewed as a specious undertaking.

The next morning, word came to Mons from Washington that Haig would have to live with it, like it or not. Deputy Secretary of Defense Charles Duncan, acting secretary during Harold Brown's temporary absence, overruled Haig. Direct orders were transmitted from Duncan to Haig'd deputy; Haig was odd man out.

Those secret orders are described as "ambiguous" by those who have seen them. The widely respected Huyser is reported by colleagues to have been unhappy with his task. But as a good soldier, he did not complain then or now (he is currently on active duty at Scott Air Force Base, Ill.).

Haig's theory that Huyser was an instrument of U.S. pressure to drop the shah is strengthened by this fact: his mission coincided with leaked reports out of Washington the U.S. policy-makers finally had concluded the shah must go. U.S. policy at this time was that Bakhtiar could gain influence over the military and win over the Moslem radicals only if the shah were out of the picture.

Whether or not because of Huyser's carrying out his orders, there was no military coup. That did not save Bakhtiar's short-lived regime from being supplanted by Ayatollah Ruhollah Khomeine. Nor did the Carter policy achieve its stated purpose of keeping Iran's officer corps intact. While many officers were executed by Islamic revolutionaries, the chief of staff contacted by Huyser -- Gen. Abbas Gherabaghi -- is believed to have cooperated with the mullahs running the revolution.

Nobody knows whether a military coup would have brought Iran stability. There are senior U.S. Army officers who believe that, had it not been for the mission imposed on Huyser, the Iranian military would have seized power, exiled the shah (perhaps letting him return as a ceremonial monarch) and established a moderate, pro-Western regime. That theory may well understate the volcanic fury of Khomeini's followers.

The point of Haig's revelations is that the administration's plea that it could do nothing to save the shah is not the whole truth. As with Anastasio Somoza in Nicaragua, the United States contributed to the demise of a repressive authoritarian who had been a longtime ally of this country in hopes of winning favor with his successors. It is that policy, rather than the president's day-to-day conduct of the current crisis, that is most vulnerable to future investigation.