Lebanon will not become Ronald Reagan's Vietnam. But the April 18 attack on the US Embassy there suggests that Beirut could become America's Belfast. The US-sponsored security agreement between Israel and Lebanon, even if Syria finally accepts it, will do little to change basic circumstances which point to such an end.
When British troops took security control of Northern Ireland on Aug. 29, 1969, they were warmly welcomed by the Roman Catholic populace as protectors against Protestant violence. The Catholics looked to London, moreover, to institute a new political regime that would eliminate the socio-economic discrimination which made the Catholics a permanent underclass.
When London proved unable to broker a settlement between the Protestant and Catholic communities, British troops themselves fell victim to violence. For when British governments - Labour and Conservative alike - could not produce needed change, they were blamed by aggrieved Catholics for the unjust status quo. They attracted such blame by virtue of the fact that exercise of power to preserve order - even if power is exercised impartially - has the effect of underpinning the status quo.
US forces in Beirut may be headed toward the same unenviable situation. The American police presence - no matter how impartial and restrained - will be increasingly seen by Lebanese and Palestinian opposition elements in a hostile light. Like the British government in Northern Ireland, if the Reagan administration fails to generate movement on vital political issues, this failure could expose US forces to violent expressions of grievance by those demanding change, who will see the policeman standing in the middle not as a neutral agent but as the embodiment of the established order. American Marines and diplomatic personnel could then pay the price of Washington's apparent inability to foster meaningful change.
There are substantive reasons, moreover, that Washington would project such an image. The PLO evacuation from Beirut, completed on September 1, was negotiated in July and August through the intermediation of US special envoy Philip Habib. Although the accord saved Beirut from destruction, and permitted PLO fighters and leaders to escape annihilation, it underwrote the main Israeli purpose of removing the PLO from the Lebanese capital and destroying its military capabilities. The US then violated its pledge to PLO chairman Yasser Arafat to guarantee the safety of Palestinian civilians left behind in Beirut. Arafat's angry statement that ''American credibility disappeared in the alleyways of Sabra and Shatila and among the blood of the innocent'' has more validity than Reagan dares admit.
The administration's sponsorship of the current Lebanese government is cut from the same fabric. A strong central government is sorely needed in Lebanon; and it is the proper aim of US policy to help construct an effective state apparatus. But under present circumstances, this is not a politically neutral act. The central Lebanese government which the US supports harasses and brutalizes helpless Palestinians and represents - whatever the temporary ''unity'' behind Amin Gemayel - only the conservative side of a fractionated polity.
Finally, the apparent collapse of the Reagan peace plan, announced with such fanfare and high hopes on September 1, could create additional peril for US troops. For Arab critics are bound to attribute Reagan's failure to Washington's partiality toward Israel.
The President's eleventh-hour ''ban'' on F-16 sales was mostly for show: Israel is already so strong militarily that the F-16s could have only negligible impact on the Israeli position - especially since the planes were scheduled for 1985 delivery. If Reagan were serious about pushing Israel he would have withheld the economic aid which is vital to sustaining an Israeli ''economic miracle'' through which high military expenditure, rampant inflation, slumping exports, and massive foreign debt have coincided with unprecedented consumption of luxury goods (including vacations abroad), low unemployment, high social spending, and universal indexation that buffers nearly everyone from rising prices.
The Israeli withdrawal accord will in no sense contribute to vitally needed movement. It enlists the Lebanese Army in the work of guaranteeing Israeli security, largely on Israel's terms. The presence of Israeli troops in Lebanese-Israeli joint patrols means that Israel did not even commit itself to complete withdrawal. By agreeing to such terms, the Lebanese government moves more to the right, making accommodation with internal opposition forces more problematical. Coupled with Shultz's announcement that the administration would now go ahead with the F-16 deal to Israel, the security agreement relieves Israel of any pressure to go further toward a comprehensive settlement.
The immobilism of the administration's diplomacy produces a situation where, the longer the Americans stay in Beirut, the more they are likely to become associated with the status quo that could come under increasingly violent attack. Reagan would then face the alternative of stepping up the American commitment or withdrawing from Lebanon in utter defeat.