A $22 million monument rising from the flat plains beside the Tigris River celebrates the battle of Qadissiya, the Arab triumph in 635 A.D. that broke the back of Persian rule over Mesopotamia.

For the families that drive 30 miles southeast from Baghdad to see a panoramic painting of the battle and picnic in the adjoining park, the monument has more than historical interest. Iraq's Arabs are again locked in a struggle with Persia, more than 13 centuries later, and the new war is called "Saddam's Qadissiya" to suggest that President Saddam Hussein is headed for an equally epochal victory over Iraq's enemies in what is today the Islamic Republic of Iran.

For Americans used to viewing the Middle East in terms of the Arab-Israeli dispute over land, it is difficult to look beyond the immediate causes for Iraq's 18-month-old conflict with Iran--jurisdiction over the Shatt-al-Arab waterway linking the Tigris and Euphrates to the Persian Gulf, for example. But a visit to Iraq leaves a strong impression that other forces are also contributing to the struggle, and may indeed be predominant.

Judging from Iraqi comments and public declarations, as well as observations of diplomats in Baghdad, these forces include traditional enmity between Persians and Arabs, the historical challenge of Shiite Islam to Sunnite Moslem rule and resistance by fundamentalist mullahs in Iran to the self-assigned mission of Hussein's Arab Baath Socialist Party to secularize and modernize Iraq and the Arab world.

Against this background, efforts to bring the conflict to a halt by mediation aimed at generating mutual concessions and compromise seem extremely difficult.

"We are defending the values of the modern world against a barbaric onslaught, and we are confident that the future will prevail over bygone days," Iraqi Undersecretary of Information Abdul Gabbar Mohsen said in a recent interview in his Baghdad office.

An Asian diplomat with a broad range of contacts among Iraqi officials expressed a similar view, but more succinctly. "This war is not really about the Shatt-al-Arab," he said. "It is about the Islamic revolution."

Iraqi officials stress that the three-year-old Iranian revolution--described by one Iraqi as a "sickness" that cannot be allowed to spread--threatens the entire Persian Gulf area. They proclaim their war is being fought on behalf of the whole Arab nation, a view recently endorsed by King Hussein of Jordan, Iraq's closest ally. He said Baghdad had become "the front line" against subversive designs of Iranian mullahs.

Iraq is particularly vulnerable to such subversion. It has a long border with Iran, a war front since September 1980. In addition, its Shiite Moslem majority, estimated at about 55 percent of the country's 14 million inhabitants, is ruled by an authoritarian government led by Sunnites.

Under Hussein's Baathists, Iraq practices a tolerant shade of Islam. Alcoholic beverages are freely available, women drive unveiled down Baghdad streets as aggressively as men, and women students in European dresses at Al Mansuriya University mix easily with their male classmates. Young couples can be seen snuggling on park benches beside the Tigris in Baghdad or smooching in cars parked on side streets.

Such liberties are anathema to the Islam preached by Iran's Ayatollah Ruhollah Khomeini, whose mullahs have gone so far as to segregate the ski slopes north of Tehran. The Iranian-style appeal for adherence to Koranic strictures is among the drawing points for the main Iraqi Shiite opposition group, The Call, with offices in Tehran and, Iraqi officials say, money and weapons from Iran to stir up subversion in Iraqi towns.

Although The Call's influence is difficult to assess in Iraq's closed system, Iraqi officials have taken the danger seriously enough to expel thousands of Shiites of Iranian origin in arrests that began before the war broke out and continue periodically even now. The execution of one of Iraq's leading Shiite mullahs and a ruthless crackdown on Call cells have resulted in its disbanding here, at least for the present, according to diplomats in Baghdad.

The role of Shiite Islam as a vehicle for religious and political protest strikes a familiar note. In the late 7th century, recently converted Moslems called Mawalis turned to Shiism to express social and economic grievance against the established order of the Umayyad Dynasty centered in Damascus.

Aside from religious differences, Persians and Arabs have been ethnic rivals for centuries. In this spirit, Iraqi newspapers and television announcers rarely use the word "Iran" in war communiques or battle reports. Instead, they almost always say "the racist Persian enemy."

Hussein underlined the historic enmity in a recent speech to troops departing for the front, charging Iran with "reviving the negative aspects of history."

"Who else ruined Babylon?" he asked. "Who else cooperated with the Jews throughout history? They have cooperated with the Jews to destroy Babylon and cooperated with them to harm Iraq and the Arab nation. Today they cooperate with the Jews. While Tehran is launching aggression against the land of Iraq, the Zionist entity launches its raids against Iraq and sends arms, expertise and equipment to Tehran to maintain aggression against Iraq."

Historically, Hussein apparently was referring to the conquest of Babylon, 80 miles south of Baghdad, by the Persian Emperor Cyrus the Great, who freed the Jews in Babylon and permitted their first return to the land of Israel in the 6th century B.C.

The Iraqi leader's other references were to cooperation between Israel and Iran under the late shah, with the shah providing petroleum to the Jewish state and Iranian and Israeli intelligence services sharing information on potential Arab enemies. More recently, Israel has provided military assistance to the Khomeini government in its war with Iraq, according to Iranian sources, and Israeli warplanes bombed Iraq's French-built nuclear reactor in June.

The Iraqi government is also angered by the friendship of Syria, a fellow Arab nation, with Iran. The Syrian leadership of President Hafez Assad is composed predominantly of members of Assad's own Alawite sect, an offshoot of Shiism.