Coming as a result of heavy American diplomatic pressure, Israel's agreement Dec. 19 to begin negotiations on pullback of its army in central Lebanon is the first step necessary if the Reagan peace plan is to be implemented.
As small as this first step may seem, it leads in important directions for all concerned:
* Israel wants this to result in a Camp David-style peace treaty with Lebanon.
* Lebanon wants border-to-border sovereignty over its own country.
* Arab powers want an independent Palestinian state to follow.
* And the US wants a comprehensive Middle East peace based on President Reagan's Sept. 1 proposal.
The Israeli Cabinet approved an agreement, devised by US envoys Philip Habib and Morris Draper, to begin talks with Lebanese officials. The aim initially will be withdrawal of Israeli troops from the area of the Beirut-Damascus highway and the suburbs of Beirut. The Israeli Army would fall back to the vicinity of Damour, a coastal town 10 miles south of Beirut. Syrian forces would simultaneously be withdrawn to the Bekaa Valley.
The Dec. 19 breakthrough came after President Reagan one day earlier called on Israel, Syria, and the Palestinians to remove their ''armies of occupation'' from Lebanon. Mr. Habib apparently carried a strong message from Reagan to Israel seeking concessions.
Clearing the way for negotiations to begin was Israel's relinquishing of its demand that negotiations with Lebanon be held part of the time in the controversial venue of Jerusalem. Lebanon, according to the Israeli Cabinet secretary, Dan Meridor, agreed to discuss political as well as military relations with Israel. Lebanese foreign minister, Elie Salam, made no mention of direct negotiations.
Pullback of Syrian and Palestine Liberation Organization forces is the next step, along with interposition of Lebanese and multinational troops in the vacated area. Mr. Draper Dec. 19 embarked on a shuttle mission to Damascus and other capitals to win this point. Mr. Habib returned to Washington to brief the administration on what was accomplished.
If Syria and the PLO show flexibility similar to Israel's, the tough negotiations will then begin. The US will try to achieve full Israeli, Syrian, and PLO withdrawal from Lebanon, with security guaranteed to all concerned.
Only after that will the US, Israel, and the Arab powers be able to turn their full attention to President Reagan's proposal to create a Palestinian homeland in the Israeli-occupied West Bank and link this homeland to Jordan.
It is in the arrangement of complete Israeli, Syrian, and PLO evacuation of Lebanon that real problems will arise. The Lebanese view is that low-level military negotiations with Israel should take place to remove Israeli troops from southern Lebanon. The Israeli view is more sweeping.
Prime Minister Menachem Begin and Defense Minister Ariel Sharon - architects of the Israeli invasion of Lebanon last summer - see the occupied south of Lebanon as a valuable bargaining chip similar to the Sinai peninsula. Just as in Camp David, Israel traded land (the Sinai) for peace with Egypt (guaranteed by the US), Sharon and Begin want to trade southern Lebanon for a peace treaty with Lebanon.
There is an economic bargaining chip in this as well. A condition of Camp David was that Egypt sell Israel oil from its wells in the Sinai. A condition of peace with Lebanon almost certainly would be agreement to sell Israel water from the Litani River. This water is currently little used by the Lebanese, and Israeli economists see it as a resource dearly needed by Israeli agriculture to allow further development of the West Bank and the Negev Desert.
With Israeli troops out of Lebanon, the Reagan plan could win wider regional support. Israel would have the iron-clad security it desires. Egypt, which has frozen its peace talks with Israel due to occupation of southern Lebanon, would be ready to support an extended peace process. And based on the Lebanese experience, Jordan and the PLO would perhaps trust the US to guarantee their security as they take the controversial step of compromising with Israel and creating a West Bank-Jordanian entity.