Part Cromwell's New Model Army, part Napoleon's youthful generals, the Iranian war machine is propelled by a faith that Western analysts liken to that of the French and Russian revolutions, and Iranians say is today's equivalent of the militant, all-conquering Islam of the 7th century.

Military specialists watching the war with Iraq from this side credit Iran's most impressive victory to date--last month's expulsion of the Iraqi Army from the central sector--to daring, unorthodox and youthful leadership, meticulous planning, battlefield patience and a willingness to sacrifice lives.

But the unsung hero of what may be one of the most ironic chapters in military history is the late shah Mohammad Reza Pahlavi, whose prodigal provisioning in modern arms, spare parts and ammunition has allowed those who destroyed his rule to repel the invader.

The shah's insistence that his largely American-supplied armed forces go first class--and receive training on the most advanced weapons system available in the West--contributed to the loss of his throne to anti-Western fundamentalists. Yet that insistence has paid off for Ayatollah Ruhollah Khomeini's Islamic republic, as pilots, technicians and helicopter crews clearly have outperformed their mostly Soviet-equipped Iraqi enemy, according to specialists.

Iraqi intelligence, according to reliable informants, is convinced the Iranians are applying ultra-modern tactics. But analysts here say the edge comes from a combination of equipment and young officers with the initiative that brigade commanders in most armies--especially Arab armies--have had drilled out of them in staff college.

The military has been twice purged of its general officers since the revolution three years ago, with many veteran lieutenant colonels and colonels shoved aside and forced into early retirement. Last fall the overall military commander, Gen. Valliollah Fallahi, the Air Force commander and many other senior officers were killed in a plane crash.

Into key commands have moved 26- and 27-year-old brigade commanders--ambitious, motivated and imbued with that Islamic dedication which is the regime's hallmark.

These commanders make "prompt battlefield decisions which an older officer would reject," one military specialist said, "and the Iranians think nothing of using an F14 air interceptor for a ground support mission which any Western general would find anathema."

Convinced that those who die in a holy war have passports to paradise, constantly encouraged by religious commissars along the front lines, the Iranians willingly have sacrificed lives to achieve their objectives.

Iran has published no official casualty list. But foreign analysts believe as many as 80,000 Iranians have died in the fighting. The principal objective remains inflicting maximum casualties on Iraq, which has only a third of Iran's population and whose losses are believed to be considerably less.

Iran's human-wave infantry tactics, at times involving the dispatch of fervently religious volunteer youngsters to cross minefields as they clutch Korans to their breasts, are carefully coordinated with a meticulous study of enemy positions and armaments. Analysts say that special units are assigned to every machine-gun nest, artillery fire base or tank division.

In another example of massive applied manpower, and the kind of patience that allowed students who seized the U.S. Embassy in 1979 to reassemble shredded American secret documents, the Iranians detailed a small army to map out the vast storage areas containing 20 to 30 million aircraft and helicopter spare parts acquired under the shah and theoretically "lost" when the prerevolutionary computer storage system all but ceased functioning.

It is that kind of attention to detail that helps explain the slow pace of Iran's offensive that began last September and has produced major victories at roughly three-month intervals.

In the process, the Islamic revolution has smoothed over rivalries and fused together the regular professional armed forces with the Pasdaran, or Revolutionary Guards who act as the ruling mullahs' military arm, and tens of thousands of volunteer youngsters.

What specialists describe as the "Pasdarization of the army"--a melding that has meant in some cases that revolutionary guards command Army units and vice versa--has been carried out by Col. Sayyed Srirazi, at 34 the ground forces' commander.

Under his leadership the numbers of Revolutionary Guards has come to rival that of the Army ground forces themselves.

Considered to be ruthless and ambitious, Srirazi is sometimes described as Iran's "new Napoleon." He was catapulted from captain to commander of the 28th Division in Kurdistan in the aftermath of the 1979 revolution.

Relieved of his command for the alleged killing of arrested Kurdish villagers, Shirazi wangled a meeting with Khomeini and returned to command both the 28th and the other division, the 64th, in Kurdistan.

While such young career officers have been placed in key command positions, the ruling mullahs are making sure that their younger comrades will be up to standard religiously. The service academies require their first-year cadets to spend three months with the Revolutionary Guards to screen their Islamic credentials.

The Iranian edge in man and machine power, combined with aggressive, motivated leadership, has led foreign analysts here to rearrange the classic odds in the crucial forthcoming battle for Iran's southern oil fields in Khuzestan Province still occupied by Iraq.

On paper, the Iranians can muster perhaps 650 tanks--albeit superior British-built Chieftains and American-manufactured M60s and older M48s--against 3 1/2 Iraqi divisions equipped with 400 Soviet-built T62 and Korean War-vintage T54s and T55s.

The Iraqis have many more tanks and armored vehicles in reserve than Iran. Standard military doctrine is that the Iranians, as the attacking force, should have a three-to-one advantage in such vehicles. Yet specialists are convinced the Iranians may win in what should prove to be the war's decisive battle without that advantage.

The exact timing of the anticipated Iranian offensive in Khuzestan is being kept a military secret, with specialists weighing the desirability of moving fast while the enemy's morale is still reeling from the central sector defeat, as well as political considerations for a quick end to the war.

Some analysts wonder whether the Iranians might, despite assurances to the contrary, invade Iraq proper to soften up the southern sector or hasten the end of Iraqi President Saadam Hussein's regime.