The Syrian peackeeping corps in Lebanon, having subdued the Palestinians, is now moving to curb the strength of the Maronite Christian militias, which relied on Syrian support to end the fighting in their favor a few months ago.

The extremist defenders of Maronite Christian nationalism are the major internal obstacle to the Syrian design for this exhausted country. Syria is apparently trying to bring the whole of Lebanon under a strengthened central government of President Elias Sarkis, who took office in the war's final spasm of violence with no power base and no battalions.

The Syrians apparently plan to keep their troops here for another year or 18 months to pacify the country and consolidate the hold of Sarkis' government, which will be pro-Syrian.

The bulk of the Syrian forces, deployed as police, turn out to be former illegal laborers here, ordered out of Lebanon by Damascus during the fighting. Besides being familiar with Lebanese streets and people, these recruits enable Syria to keep most of its fighting units at home, far from the risks of political contamination in Lebanon. The Syrian force, however, is spearheaded by crack troops, managed by precise Syrian commanders.

As these veterans of the Golan Heights front against Israel moved onto the Christians' home turf, the militia fighters melted away. "We're amateurs, they're professionals," a Phalangist said.

The Syrians have now ordered the closure of the 32 private Christian ports - mostly coves or jetties built with looted cranes - that handled contraband from Scotch to Sherman tanks. The private airstrips are to be shut next, followed by the party radio stations.

Sarkis is about to fill key jobs - new heads of security, police and army - and his strength will be indicated by his ability to impose his own candidates.

The go-slow approach, instinctive with Sarkis - as cautious as his supporter, Syrian President Hafez Assad - is criticized here by many officials and diplomats who argue that he should have moved ruthlessly at the outset.

The old confessional problems are dogging reconstruction. The National Aid Council is impeded by sectarian wrangles because Sarkis has nominated a Maronite Christian to lead it.

It is clear that the oil-rich Arabs are going to dole their aid carefully, starting slowly with the Beirut port. So Sarkis' plan is to move step by step, to foster pacification and the economic revival - taking Lebanese minds off the political vendettas that decimated this country.

In his determination to give Lebanon a more Arab-oriented system, Sarkis, himself a Maronite but a reformer, has faced the Christians with an agonizing choice, which has implications for Christian Arabs through the Middle East.

The Maronites, who waged civil war to resist the Arab and Moslem hegemony supposedly spearheaded by the Palestinian guerrillas, ironically find themselves dependent now on Syria, a traditional threat to Lebanese Christian independence.

A fierce debate now divides the Maronites on how to react. The Phalangist Party, probably the fighting majority, argues that Lebanon's Christians must become part of the Arab world, that Lebanon must be a Syrian protectorate, that this would enable the Christian to be accepted in the Arab and Moslem world. The Phalangists support Sarkis.

Sarkis, however, faces a challenge from the Maronite Christian extremists, who have a latent ally in Israel, which is loath to see the Lebanese Christians drawn closer into the Arab world.

In a caucus of Maronite leaders, the extreme elements - including former President Camille Chamoun and the monks who are the Maronite intellectuals - promoted a plan for radical "decentralization" of the new Lebanon into three ethnically homogenous zones: a Christian Mt. Lebanon, Sunni Moslem north and Shia Moslem south.

Each region would have its own government and army, while a weak central government in Beirut would be run by a presidential council. This would eliminate the confessional frictions that paralyzed Lebanon, it is claimed.

The plan mentions British devolution as a precedent. The trouble is that the Maronite map points to the creation of a Christian ministate, which would occupy Lebanon's heart and ultimately attract the hostility of its Arab neighbors.

The idea of local self-government has no tradition in the Arab countries, which are mostly former colonies ruled by military regimes. It remains to be seen whether Sarkis can persuade Syria to permit some measure of local initiative in return for a Syrian protectorate over Lebanon.