United States involvement in Lebanon's violent troubles seems headed for a crucial turning point. At the weekend Syria threatened to retaliate against any American weaponry on land, sea, or air that fires on Syrian forces.
This strong language from a Syrian military spokesman followed unprecedented US naval bombardment late Friday night and early Saturday morning. A US marine spokesman said the salvos hit ''deep into Syrian-controlled Lebanese territory'' against artillery positions which had shelled the American ambassador's residence and the Lebanese Ministry of Defense.
US marine spokesmen insisted the US move was strictly defensive of US interests - a six-man team of US advisers to the Lebanese Army operates out of the Lebanese Defense Ministry. But many Lebanese observers saw it as a move to boost the fighting morale of the embattled Lebanese Army which over the weekend escalated the conflict against rebellious Lebanese factions after the failure of a US-Saudi mediating a effort to achieve a cease-fire.
With Congress and President Reagan locked in debate both over the power to assign marines to Lebanon and the justification of their presence there, the US military action raised new questions about how far the US definition of ''defensive'' action in Lebanon will expand.
The US role in Lebanon has widened sharply since Israel's Sept. 4 withdrawal from the central Lebanese Shouf mountains sparked fierce fighting between Druze Muslims on the one side and Christian militiamen and the Lebanese Army on the other. The Druze demanded a new, more favorable Christian-Muslim political power balance for the Muslims before the Lebanese Army, which they distrusted as pro-Christian, be allowed into the Shouf to take abandoned Israeli positions.
But the Lebanese government insisted that the Lebanese Army first be allowed to establish central control over the Shouf before political negotiations began.
On Monday Sept. 12, 2,000 additional US marines arrived, remaining offshore to bolster the 1,400 marines already serving with international peace force in Beirut.
In order not to invoke the War Powers Resolution - which gives Congress the right to limit the marines' stay to 60-90 days - the marines were forbidden to engage in hostilities. However, the definition of ''defensive'' action has steadily widened.
Last week President Reagan gave US marines in Beirut the right to order air and sea strikes to defend the international force and, in some cases, to respond to attacks on Lebanese Army units, if a unit near marine positions was in danger of being overrun. Marine spokesman Major Robert Jordan says the marine mandate also extends to protection of the US diplomatic missions and all other US interests and personnel in the area.
The US naval bombardment came at the end of a week when Western diplomats and the news media in Beirut were predicting that US and Saudi mediators were about to produce a ceasefire in the Shouf. These predictions continued as late as Thursday, as Saudi Prince Bandar Ibn Sultan shuttled from Damascus to Cyprus and Beirut. Diplomatic sources in Washington were more gloomy.
On Friday morning, Lebanese were shocked to wake and find the Saudi effort had failed and the Lebanese Air Force had attacked - in their first sortie since 1976 - what they said were Palestinian guerrillas allied with the Druze who were trying to infiltrate over a ridge toward Beirut. Of six planes in the fledgling air force, one was shot down and another damaged.
The attack was in keeping with Lebanese government charges that Palestinian guerrillas expelled from Beirut last summer are backing up the Druze, a claim which Druze spokesmen deny. While journalists have seen small groups of Palestinians in the mountains, it has been difficult to verify how strong their role is or whether there is any coordination with the Druze.
The US naval action came several hours after the first Lebanese air raid. US marine spokesman Jordan labelled the move ''an appropriate defensive response.'' When asked why the Lebanese Army could not protect the Defense Ministry he replied that it was ''busy in other areas.''
In fact, the marine mandate to protect Lebanese units near marine positions gives guaranteed ''defensive'' protection to Lebanese Army rear lines located near marine positions at Beirut International Airport.
If negotiations toward a ceasefire in the Shouf can be revived and ultimately can succeed the indirect links between marine ''defensive'' moves and Lebanese Army advances may prove of small consequence. But if the talks continue deadlocked, the broadening of the definition of ''defensive'' could provide a spark to further escalation and deeper US involvement.