The commandant of the United States Marine Corps asserted to the US Secretary of Defense in a formal letter dated March 14, then made public on March 17, that his men on duty in Lebanon have been persistently ''harassed, endangered, and degraded'' by Israeli troops.
The government of the US gave a copy of the letter to the Israeli government, which replied on March 18 recognizing that there has been friction between US and Israeli forces in Lebanon but blaming it on the alleged refusal of US forces in the area to permit liaison between the two forces.
Two days later the commander of the US Marine force in Lebanon stated publicly that Israeli forces have been endangering both US Marines and local civilians by ''gross lack of fire discipline.''
These episodes are the latest surface manifestation of friction which has persisted in Lebanon between Israeli and US forces almost from the beginning of the US military presence there last September.
The record shows a series of incidents in early January (Jan. 5, 6, 8, 11, and 17). In all of these, Israeli forces attempted to pursue Palestinians who had entered American lines to escape. In each case the Marines refused to permit the Israelis to go through their lines.
On Jan. 21 there was another incident when the marines reported that an Israeli jeep ''nudged'' an American officer and Israeli tanks pushed through barbed wire marking the boundary of the marines' position.
On Jan. 25 the Pentagon in Washington said it was making an ''analysis'' of the earlier incidents.
On Feb. 2 occurred the most dramatic and widely reported incident. An Israeli tank attempted to push through a US Marine roadblock. US Marine Capt. Charles B. Johnson loaded his revolver, jumped up on the Israeli tank, and stated that they would pass ''only over my dead body.'' His action was commended by President Reagan.
On Feb. 5 unnamed ''leaders'' at the Pentagon were quoted as charging that Israeli Defense Minister Ariel Sharon (since replaced) had been pursuing a policy of trying to ''discredit'' the international peacekeeping force in Lebanon.
On Feb. 10 US Secretary of Defense Caspar Weinberger rejected an Israeli proposal for sharing information gained by Israel during its 1982 fighting in Lebanon. Secretary Weinberger stated that the conditions attached by the Israelis to the deal were unacceptable. One condition was that the US must not pass along the information to its NATO allies. (Israel dropped the conditions on March 21.)
On Feb. 27 President Reagan talked about the possibility of US armed forces protecting Israel's northern frontier. The Israeli government immediately rejected the idea.
Apparently there were further incidents during the first and second weeks of March although the accounts of these have not yet been reported in detail. Whatever they were caused Gen. Robert H. Barrow, commandant of the US Marine Corps, to state in his letter that ''it is time for firm and strong action to demonstrate to the Israelis that a role as a peacekeeper does not presume weakness.''
All of this points up a conflict of interest opening between the US and Israel.
The US has sent its troops into Lebanon not to support and strengthen Israel's position in Lebanon, but to support and sustain the government of Lebanon in its own effort to regain control over its entire territory. US Marines are not there to help Israeli forces run down escaping Palestinians but to protect Lebanon and Lebanese from violence from all outsiders. US policy is not supporting Israel in its desire to maintain a continuing military presence in southern Lebanon. It is doing what it can through diplomacy and persuasion to get the Israelis out of Lebanon entirely and permanently.
Add to the above that the President of the United States, like all his predecessors since the 1967 war, favors the withdrawal of Israeli forces from territories occupied by Israel since that war, including the West Bank of Palestine.
Right now is the moment of decision on whether US presidential policy will prevail and the Palestinians will be allowed to have a homeland of their own in Palestine or whether Mr. Begin will prevail and proceed to the annexation of the occupied territories.
The President is pursuing a policy which would show the US as a fair intermediary between Israel and the Arabs. Mr. Begin is pursuing a policy which would give Israel sovereignty over all of Palestine, dominion over a sullen and irreconcilable Arab population, and endless hostility between itself and the rest of the Arab world.
The two policies are incompatible. The incompatibility is showing along the margin in Lebanon where Israeli forces are supporting one policy and the US Marines a contrary policy.