The Israeli drive into west Beirut may well prove to be a test case for the credibility of the Reagan peace proposal for the Middle East.
The United States has called for an immediate withdrawal of Israeli forces from the Lebanese capital.
If the Reagan administration cannot succeed in persuading the Israelis to withdraw quickly from Beirut, that failure could cast doubt on the ability of the United States to guarantee implementation of the recently announced Reagan plan for resolving the much larger Arab-Israeli conflict.
This helps to explain the unusually strong statement issued by the State Department on Sept. 16. John Hughes, the State Department spokesman, said: ''It appears that Israeli forces have now moved into strategic positions in west Beirut. This is contrary to assurances given us by the Israelis here in Washington and in Israel.
''It is a clear violation of the cease-fire agreement,'' said Mr. Hughes. ''There is no justification for a continued Israeli presence in west Beirut. We are calling for an immediate withdrawal.''
From a military point of view, the Israeli move into Beirut, according to most experts here, does not change the balance of forces. But the occupation of an Arab capital is seen as a humiliation for the Arab leaders.
The Israelis argue that the move was required to prevent chaos following the assassination of Lebanon's President-elect, Bashir Gemayel. Lebanese officials say they had the situation under control and the Israelis were not needed. Much may depend on the ability of the Lebanese to elect a new president and put together a coalition government. In the US view, the best way to do that is not under the barrels of Israeli guns.
Reports from Israel indicate that Prime Minister Menachem Begin told American special envoy Morris Draper on Sept. 15 that he had ordered the Israeli Army into west Beirut because the Lebanese Army had fallen apart following the killing of President-elect Gemayel. Mr. Begin was quoted as saying that the Israelis feared a resurgence of remnants of the Palestine Liberation Organization in the city, who, together with their leftist Muslim allies, might block the formation of a new central government.
Lebanese officials denied that this was the case. They have now gained American support for their position that the Israelis are not needed, and that the Lebanese Army should be given a chance to consolidate its hold on the city.
''I don't know that the Israelis want to remain in Beirut,'' said William B. Quandt, a Middle East expert at the Brookings Institution, who dealt with the Israelis as a Carter administration official. ''To their credit, the Israelis may have been jittery after the death of Bashir Gemayel.
''But now that it seems less likely that there will be chaos - the Muslims have been very restrained - it seems to me we can say, we understand your being jittery but you've got to get out.
''If the Israelis say 'no,' then we've got to consider the traditional ways of exerting pressure on Israel.''
The latter option is a distasteful one for American officials. Secretary of State, George P. Shultz, has apparently decided that any direct pressure, such as a cut in economic or military aid to Israel, would probably be counterproductive.