Arms shipment to Tehran could enhance next Iranian offensive. Military experts say Iran has doubled its air sorties recently

American arms shipments will greatly enhance Iran's ability to launch a threatened final offensive against Iraq, according to military experts. New United States antitank weapons and antiaircraft missiles will by themselves provide much added punch. Perhaps more important, US allies may now decide it's all right for them to broker arms with Iran, too.

``The revelations of the last couple of weeks will open the floodgates,'' predicts a former high Reagan administration official.

Recent increased activity by the Iranian Air Force shows how new supplies of spare parts are affecting the Persian Gulf war, according to this source. Sorties by Iran's F-14 fighters, one of the most advanced and complex warplanes in the world, have doubled in recent months.

Now more than six years old, the Iran-Iraq war has become the most brutal conflict since World War II, with more than a million casualties. US military officers estimate the war has killed or wounded 3 percent of the Iraqi population and 1 percent of the Iranian population.

The last major move of the war was in February, when a well-executed amphibious attack by the Iranians captured Iraq's vulnerable Faw Peninsula. Since then Tehran has built up its forces and talked constantly of another big attack, a final offensive deeper into Iraqi territory. Iraq, in retaliation, has escalated its air war against such targets as Iranian oil terminals.

Against this background, the weapons secretly shipped by the Reagan administration could provide much needed aid to Iran, experts say. The type of weapons provided addresses Iran's two major military problems:

Air defense. Iran has been badly battered by Iraq's superior Air Force. According to the International Institute for Strategic Studies, Iran has but 68 combat-ready aircraft, while Iraq has 500.

Iran's air defense, which until now has been dependent on Soviet-made SA-7 missiles, has obviously had problems, says a private analyst of the Mideast arms balance. Throughout the summer Iraqi planes flew over such vital Iranian installations as the Kharg Island oil terminal with impunity.

But the 235 Hawk antiair missiles that the US has provided Iran have a range of 25 miles. They can reach the high altitudes Iraqi pilots favor. Indeed, analysts note that Iraqi air attacks tapered off in September and October. The air war resumed in November, however: On Tuesday Iraqi jets struck Iran's Larak Island oil terminal at the mouth of the Persian Gulf.

Anti-armor weapons. On the ground, the biggest problem of Iranian commanders is the need to counter Iraq's vast superiority in armored weapons. Iran does have some 1,000 main battle tanks, mostly various Soviet models, but Iraq has 4,500, and they are settled along the front in well-protected defensive positions.

The 2,000 TOW (Tube-launched, Optically tracked, Wire-guided) anti-armor missiles sent to Iran by the US are ideal for attacking dug-in tanks. ``They are a very lethal weapon,'' notes Anthony Cordesman, a Mideast arms expert with the Eaton Corporation.

TOWs could help Iran successfully to roll back Iraqi defenses around the threatened town of Basra. But 2,000 is only enough missiles for one or two large battles, experts note, not a whole war.

Some Western analysts had expected Iran to launch a major attack in September or October. Half a million Iranian troops had been mobilized and were poised at the front. But this much-discussed ``final offensive'' has yet to occur. One reason may be that the Iranian Army just is not ready. ``It takes Iran a long time to mass enough arms for an attack,'' says Shaul Bakhash, a history professor at George Mason University.

Iran may simply be rattling its sword to keep Iraq nervous and stretched thin along the whole central front.

But there is also apparently a power struggle in Tehran over broad questions of military strategy. The Revolutionary Guards, fervent believers in Ayatollah Khomeini, are bickering with the professional army over what sort of attack to launch. The guards favor human-wave tactics; professional officers prefer more traditional military planning.

``There is a real debate going on over the prosecution of the war,'' says Gary Sick, a National Security Council staff member under President Carter.

An early spring attack, when the ground along the front is swampy, might favor Iran's more lightly armed divisions. By then Iran would have stored up more arms - such as the jet fighters it has reportedly purchased from China. US allies, seeing the debacle of US arms sales to Iran, might well have decided to ship Khomeini weapons by spring. ``It was difficult enough to keep our allies from shipping Iran arms before this,'' says the former high Reagan official. ``They were continually threatening to fall off the wagon.''