President Hafez Assad of Syria has emerged from an unusually long silence to ridicule President Sadat's peace initiative - "It ended before it started" - and to warn Israel to stop "fishing" in the troubled waters of Lebanon.

With that back-of-the-hand swipe to his Arab brother Anwar Sadat and his mortal enemy Menachem Begin, Syria's strongman laid out a scenario for Mideast and Lebanon peace in an exclusive interview with us. It was his first after three months of discreet quiet during Israel's occupation of southern Lebanon and Sadat's fading peace mission.

A militant force who must be dealt with, Assad told us on the question of overall Mideast peace, Sadat should concede what "everyone in the world knows," that his mission to Jerusalem has now boomeranged, leaving Jerusalem united and the Arabs divided.

"From whom is Sadat trying to hide this secret?" Assad asked with a twinkle. The Arabs now must get their act together because only an Arab World "with a material [military] basis of rough equality with Israel" can negotiate peace with Israel. "It is not necessary to go to war," Assad said, "but Israel will answer only to strength, never to such niceties as a trip to Jerusalem."

The United States is perilously close to underwriting a separate Israeli-Egyptian peace under the "guise" of a comprehensive settlement, he said. That would extend and deepen the Arab split and doom any early settlement.

The U.S. proposal for a transition period covering return of the West Bank to Arab sovereignty with Jordan, or as a Palestinian homeland or whatever Washington's current West Bank fad, is simply to "describe a non-solution as a solution." But Assad did not absolutely rule out a transition period.

Although he came away from his meeting with President Carter in May 1977 "with a good impression," even Carter seems unable to prevent israel "from controlling your foreign policy in the Middle East," Assad said. "We hope this may change, because as of today Israel, not the United States, dictates decisions."

Syria has accepted the principle of U.N. Security Council Resolution 242, tantamount to accepting a permanent Israel. But Assad is blunter about dealing with Israel, and more realistic about the Jewish state, than Egypt. In our interview, however, bombast was absent. The tall, mustached 48-year-old Syrian leader showed emotional flashes only on the vexing problem of Lebanon.

He told us that Syrian intelligence has uncovered specific "contacts" between Israeli agents in the Beirut area and the Syrian-hating Christian Phalangists - right-wing factions engaged in a twilight war with Syria's 30,000 frontline troops.

Israel's exploitation of that hatred among Phalangist militia in the south is well known. But Assad was saying something different. He was accusing Israel of stirring up the northern Christians and inciting them to attack Syrian forces.

"We have received news that Israel contacts have been fishing among some Phalangist elements to carry out many actions and obstructions against our efforts to establish security in Lebanon," Assad said. "They are pushing some of the phalangists into attacking the all-Arab [mainly Syrian] peace force."

Assad swears he will not be driven out of central-northern Lebanon by Israeli propaganda so long as the legal but powerless Lebanese government asks him to stay. He laughed at efforts to equate Syrian occupation with the Israel invasion of southern Lebanon. He noted that Lebanon asked Syria in, Syria and Lebanon are members of the Arab League, both are in a state of beligerency with Israel, both are Moslem-majority states.

"If Syria gave up and left, the future, I assure you, will not be better, but worse," he said. He warned Israel to cease exploiting the tragedy afflicting what used to be the brightest jewel in the Middle East: the industrious, bi-religious state of Lebanon now torn in civil war by the presence of hundreds of thousands of Palestinian refugees who became a state within a state.

"This role that the Israelis are playing will reflect adversely on Israel for a very long time," Assad prophesied. "The effects will not be limited." He did not explain, but may have been looking ahead to what many experts here see coming: conversion of Lebanon into a front-line Moslem state on Israel's northern border. Assad insisted that Israel's machinations in Lebanon are a ploy in Israel's larger game: to retain conquered Arab territory.

If Sadat's peace mission has indeed now been strangled by Israel, Assad chose a good time to emerge from self-enforced silence. He is a portent of tougher times ahead - tougher, but more realistic.