The latest outbreak of violence in Iran intensified a double confrontation. Internally, the shah, having failed to ease tensions by sweeping concessions, has been obliged by his own military to take on the dissidents in a test of strength. Eventually, the critical importance of Iran in the global confrontation between Russia and the United States now asserts itself with a vengeance.

The source of trouble is the break-neck pace of modernization imposed upon the country by the shah. Among other things he has forced economic growth at a 10 percent clip, pushed education to the point where literacy has increased sixfold in the past 25 years, nationalized land, instituted a steel industry and an auto industry, and opened schools and jobs to women on a grand scale.

Inevitably, there have been terrible dislocations, especially for young people wrenched from village life and thrown into the crazy mix of corruption and congestion, shortage and vice, which characterizes Tehran and other cities. The disaffected youth, in schools and out, have provided foot soldiers for two groups interested in undoing the regime.

By far the most important are the Moslem clergy, or mullahs. Thanks to the shah, the mullahs have lost their lands, their dominant role in education and law, and the control they exercised, as leaders of land, over the bazaars. The more fundamentalist among the mullahs have actively opposed the shah's modernization program, especially the liberation of women.

Circumstances have driven into alliance with the mullahs a small group of Marxist revolutionaries organized into tiny cells for terrorist action. At the center of this strange combination has been Ayatollah Komeini, an Iranian religious leader who has been sheltered by the radical regime in Iraq. Komeini continues to be in touch with the Islamic fundamentalists in Iran, and with radical Arabs - notably the Libyan regime of Muammar Qadaffi and the Palestinian extremists - patronized by Moscow.

About six months ago a campaign of low-level violence in Iran flared to the surface with a series of demonstrations organized against the shah by Islamic fundamentalists. The shah met those troubles, which reached a crescendo during the holy month of Ramadan, by a policy of steady concessions.

He fired the head of the secret police, or Savak. He allowed freedom of the press, and decreed that several parties could present candidates in the elections due next month. He instituted a campaign corruption that according to some reports, was due to touch even members of the royal family.

On Aug. 27, a week before the end of Ramadan, he ousted an American-educated technocrat, Jamshide Amouzegar, as prime minister, and replaced him with a traditional figure, Jaafar Sharif-Emami. The new prime minister made even more concessions to the Islamic fundamentalists. He closed down casinos and other gambling places, arranged for the dropping of the court minister - Abbas Hoveyda, who was suspected by the Islamic fundamentalists of having connected with the Bahai cult - and started consultation with at least the more moderate mullahs.

Nevertheless, demonstrations continued following the end of Ramadan on Sept. 4. In two of the protests, students tried to lure soldiers to their sides of the struggle. Faced with that kind of subversion, the army chiefs prevailed upon the shah to decree martial law before a massive protest scheduled for Friday, Sept. 8.

That day the soldiers and the protesting youth clashed in a bloody fracas. At least a hundred were killed. Now tight security prevails in Tehran, and though there has been relative calm, a single spark could set off a new wave of fighting.

For the shah has run the string on concessions. Not only has liberalization been taken as a sign of weakness by the opposition, but now the Iranian military is in the picture forcing the monarch to tough it out.

With the shah thus committed, the United States is inevitably involved. This country to some extent - and Japan and Europe far more - are heavily dependent on the Persian Gulf for oil. With weak regimes in Saudi Arabia, Pakistan and India, Iran is the only force for stability in the area.

The Russians have always looked on the shah's regime as, in Khrushchev's words, "a rotten piece of fruit" ready to fall into their hands. The more so now, when trouble in the Persian Gulf presents probably the best way to break up the developing entente among the United States, China, Japan and Western Europe.

In these conditions the United States does not have the luxury of sniffing at corruption in Iran, or playing liberal missionary on human rights, not to mention being a supersleuth on weapon sales. If the shah has seemed depressed recently - and he has - it is in no small part because of the instinct of the Carter administration for building up popularity at home by playing to moralistic suspicion of this country's allies.

To be sure, President Carter called the shah over the weekend and expressed support. But that is only a beginning of the kind of American backing required if the shah is to play in global politics the role this country's interests demand.