With the so far indecisive war against Iran entering its fourth week, Iraq pushed forward today with its biggest offensive in a determined effort to achieve a victory that would force Ayatollah Ruhollah Khomeini to sue for peace.

Having failed to impress in the field until now because of the conservatism of its military tactics, Iraq finally seems to be going all-out in an effort to overwhelm the determined, and surprisingly effective, Iranian resistance in Khuzestan Province that so far has stalled the superior Iraqi forces.

Unable to gain control of the Iranian river port of Khorramshahr after three weeks of savage siege, the Iraqi military has attempted an end run around it, ferrying troops and tanks over the Karun River 10 miles north of the port in a drive to cut off its defenders and encircle the important oil refinery center at Abadan, 10 miles farther southeast.

To put teeth into its drive toward Abadan, the Iraqis have finally begun to unleash their air force, until now held mostly in reserve. Modern bridging equipment and amphibious vehicles have been brought forward to the Karun River, which is used by the Iranians as their main defense line. And, in an effort to break civilian morale, huge Soviet-made Scud surface-to-surface missiles have been fired against Khuzestan cities in an effort to terrorize their populations.

The battle shaping up over the precarious Karun River crossing and around Abadan to the southeast is being viewed here by Western diplomats as without a doubt the most important of the war so far. For the Iraqis, it represents a gamble to wrest the sort of victory that has so far eluded its ponderously slow-moving army. For the Iranians, the battle represents its best chance to blunt the Iraqi initiative.

A victory at Abadan would allow President Saddam Hussein of Iraq to claim some of the prestige and clout in the Arab world that he had clearly sought to establish with a quick and surgical strike against Iran in the first days of the war.

Should Iraq fail to overwhelm Abadan, as the Iraqi forces have so far failed to conquer any other major Iranian strategic position, the conflict between the two nations would seem doomed to bog down into a costly and protracted battle of attrition where there would be neither victors nor vanquished.

Western diplomats here believe that Iraq could sustain such a war for six months to a year. Iran would seem less prepared for such an effort except for the stubborness and fanaticism of its revolutionary leaders -- a factor that makes any true assessment of their real staying power hard to measure.

As the battle for Abadan was shaping up, both Baghdad and Tehran showed signs of beginning to try to prepare their populations for the sort of long, drawn-out, inconclusive struggle that seemed inevitable if Iraq's offensive loses steam before the Iranian defenses of Abadan.

In Baghdad, Hassan Najafi, the governor of the Central Bank, in a move clearly aimed at psychologically conditioining Iraqi citizens for the worst, said in an interview that Iraq's foreign exchange position today was at an all-time high and that the country could maintain its present standard of living without any further oil income for a full year.

Najafi's Iranian counterpart, Ali Reza Nobari, echoed the same optimism in Tehran, proclaiming that his nation's $8 billion in reserves -- a figure believed by diplomats to be already drastically depleted -- would allow Iran to go for another seven months at its present rates of expenditure without any further oil sales.

Western diplomats in Baghdad tend to believe the Iraqi claims to be able to weather the war, even if it drags on for months. They point out that Iraq, having the advantage of planning in advance, carefully stockpiled vast supplies of weapons, ammunition, spare parts and strategic military oil stocks. sOn top of that, they still have a military supply pipeline from the Soviet Union, their principal military supplier.

Although the Soviet Union has consistently maintained its neutrality in the conflict, it is generally considered to be still providing military supplies under its treaty of friendship with Iraq. The supplies, though not massive and certainly not enough to replenish the stocks being used up daily at the war front, are reported to be arriving through the Jordanian port of Aqaba.

In Iran, military supply has been more problematical. After almost two years of being cut off from the Western world, and more recently subject to embargoes from Western nations, the Iranians find that they have few sources of military supplies or spare parts for their equipment.

However, Iran has negotiated for some spare parts and U.S. supplies fro Vietnam stocks captured when the Americans evacuated there in 1975. The stocks have been flown from Vietnam to North Korea and then over China to Iran. But they are not considered adequate to maintain a long, protracted struggle on the ground.

One sign that Iran is beginning to feel the crunch is the fact that its planes, mostly U.S.-made F4 Phantoms, have been less evident in the skies over the battle field last week. Military analysts do not believe this is the result of the decrease in oil supplies, nor does the decreased Iranian air activity reflect major losses of aircraft, despite Iraqi claims that more than 200 Iranian planes have been shot down. More likely, analysts believe, is that Iran is finding it hard to keep its sophisticated planes in the air due to a lack of trained maintenance personnel and an adequate supply of spare parts.

In the battlefield before the Abadan front today, the Iraqis claimed to be moving rapidly toward Abadan after having spanned the Karun River with pontoon bridges which allowed their tanks to cross. Military experts, however, say that the bridging of the Karun seems to be somewhat of an exaggeration. Much of the Iraqi equipment going across the river, they say, is moving on military ferries and amphibious personnel carriers that transport troops and vehicles onto the eastern side of the Karun.

From there the assembling Iraqi force is seeking to strike at Abadan, which has been ablaze from Iraqi artillery strikes for the past three weeks. Whether the Iraqi force manages to break through the Iranian defense position is another matter.

The Iranians have shown in their determined defense of Khorramshahr that their forces, made up of Revoluntionary Guards as well as crack commando and tank units, have been reinforcing in and around the Shatt-al-Arab on both sides of the Karun River for the past three weeks. So far in Khorramshahr they have totally stalled Iraqi attempts to take the city. No one believes that their defense of Abadan will be any less determined. And it is likely that the Iraqis will find themselves bogged down on the outskirts of Abadan for weeks to come as they have been on the woutskirts of Khorramshahr.

The Iranian Army has shown signs of rallying after almost 20 months of being weakened by purges and disregard by the mullahs who control Iran. As a matter of fact, Iranian President Abol Hassan Bani-sadr confirmed only several days ago that to boost the Army's effectiveness he had ordered officers released from prison so they could go to the front and fight for Iran. He told one interviewer that the patriotism of such Army officers was more important than their loyalty to Khomeini.

The Iraqi forces trying to move across the Karun River were clearly superior to the Iranian troops in the field despite their continuing reinforcements.However, the Iranians were defending their own soil and were determined to die fighting rather than be taken prisoner.Despite their weakness -- they are estimated to be outnumbered three to one in the field -- the Iranian defenders were being steeled by Moslem clergymen visiting the battle front in numbers to extol the Shiite virtues of sacrifice and martyrdom.

The most flamboyant of Iran's ruling mullahs, Ayatollah Sadegh Khalkhali, visited Khorramshahr last week to bolster morale. He said that not only were mullahs armed and fighting in the field but so were women and children and that the city's battered mosques had been turned into military command posts.

Khalkhali was quick to dismiss his forces' lack of supplies and logistical backing.

"Our Army does not need money or food -- nothing," Khalkhali declared in a radio interview heard in Iraq. "No need for letters to our mothers or our sisters -- nothing. They are here to become martyrs. They have no expectations but Islam."

As Iraq's four divisions of troops, backed with some of the most modern Soviet equipment, began to press toward Abadan, Khalkhali dismissed them with his usual bluntness:

"The Iraqi forces, imitating King Saddam [Hussein] have done nothing but engage in drinking bouts, look at sexy magazines and gamble," he sneered.

That was still a proposition which remained to be tested -- when and if the Iraqi forces decide to invest Abadan with the whole might of their vast Army moving across the plains of Khuzestan.