Efforts by Syrian and Iraqi leaders to "unify" their two countries have been given new impetus by Israeli military action in south Lebanon and the Shiite Moslem agitation in Iran, according to Arab political sources.

It remains to be seen, however, whether a three-month-old plan for unification that still is ill-defined will be any more successful than a dozen other such schemes involving various Arab countries over the last 20 years. All but one of them have failed.

Iraqi Vice President Saddam Hussein and Syrian President Hafez Assad are currently meeting in Damascus as part of a "higher political committee" formed last October when the two countries ended more than a decade of hostilities and signed a "joint charter for national action." The meeting, which opened Sunday evening, is to review progress by several subcommittees established in October to promote military, economic and political unity between the two former rivals.

Reports from Damascus quoted Arab diplomatic sources as saying Iraq and Syria plan to merge their foreign, defense and information ministries as a first step toward unification. According to those sources, the two sides hope to announce creation of one united state at a future meeting in Baghdad.

Recent Arab press reports have also spoken of plans for such a "dramatic announcement," but Arab diplomats in Beirut expressed doubt that any accord would include full-scale merger.Even if it did, other political sources said, the next question would be whether the new unified state could function any better than other previous Arab unifications which were never much more than hypothetical.

The joint charter and unification ideas came about as a direct result of the Camp David accords and the prospect of a peace treaty between Egypt and Israel. Both Syria and Iraq regard that prospect as a threat, fearing that Israel will resist giving back any more Arab land and may unleash its superior armed forces against them once it signed a separate peace with Egypt.

But another factor becoming increasingly important for Iraq is the Shiite Moslem agitation in neighboring Iran, political sources in Beirut said. The Shiite leadership in Iran has shown it can mobilize millions of people in a direct challenge to the government and play a key role in ousting a ruler as powerful as the shah.

The ruling Iraqi Baath Party and secret police are considered no less repressive than the shah's forces. But a mafority of the Moslems who make up 90 percent of Iraq's population are Shiites and could be influenced by the political activism of their neighbors, especially if the exiled Iranian religious leader, Ayatollah Ruhollah Khomeini, should decide to encourage them. Some observers feel he eventually may try to settle a score with the Iraqi government, which attempted to silence him last fall when he was loving in exile there.

"The Iraqis are frightened by the situation in Iran, and they are turning toward Syria to avoid getting caught between two fires," a political analyst said. "The Syrians are in the same boat. Both sides realize they cannot afford to remain hostile toward each other."

Syria's main worry is its border with Israel on the Golan Heights. Last week, the Syrian and Iraqi defense ministers, meeting in Damascus to work out plans for "complete military unity" under the proposed unification, toured the Golan front.The Syrians are understood to have been especially alarmed by the latest Israeli incursion into southern Lebanon and shelling exchanges across the border with Palestinian guerrillas.

Other reasons for the moves toward unification are that a relationship with a major oil producer would give Syria greater independence from Saudi Arabia, which has been bankrolling Assad's government. Unification also would allow Syria and Iraq more room for maneuver with the Soviet Union, which equips the two countries' armed forces.

Although most analysts douby that even a "unified" army would stand a chance of defeating Israel militarily, the Israelis have not lost an opportunity to underscore, and even exaggerate, the threat. Diplomats and political observers here feel that by appearing to take the unification idea seriously, the Israelis can exert more leverage on the United States for further weapons deliveries.

The Israelis are aware, however, that past Arab unification plans have had little success. Only the United Arab Emirates, a union of seven tiny Persian Gulf sheikdoms, has survived.

Many of the other unification efforts, spurred by the Arab-Israeli conflict and the quest for a long-elusive "Arab nation," never were more than theoretical.