Beirut — The United States escalated its involvement in the Lebanese conflict dramatically Wednesday, as the battleship USS New Jersey opened its massive guns against artillery positions behind Syrian-held lines. At least 130 rounds were fired in the first four-hour period, their reverberations shaking the capital.
This is only the second time US naval ships have fired other than in defense of US Marines. The first time was last September, when Sixth Fleet ships opened fire in defense of the Lebanese Army battling to keep the strategic town of Souk al Gharb.
This move was highly controversial at the time. Many diplomats and sources in the multinational peacekeeping force - consisting of troops from the US, France, Italy, and Britain - feel it helped lead to the bombing of the Marine battalion one month later, since the US was seen to have abandoned its peacekeeping job and become a party to the domestic conflict.
In effect, the New Jersey's fire puts the US in direct conflict with Syria, which controls eastern and northern Lebanon.
There are also dangerous implications for the future, since the local Muslim militias, which are backed by Syria, have pledged they will not move against the Marine contingent - unless they joined the week-old fighting.
The militias may now abandon that pledge and open up once again against the 1 ,470 US servicemen still onshore around Beirut International Airport.
The change came suddenly, since most Lebanese had focused on the stunning news that President Reagan had ordered the Marines to withdraw to ships offshore , triggering widespread panic. By midafternoon, the other part of his speech - pledging retaliation - became the talking point.
Ironically, there was little movement at the Marine base, where troops were still trying to get specifics on Reagan's announcement at lunchtime.
In effect, the marines were the last to hear the news. A Marine spokesman, Maj. Dennis Brooks, had been asleep at 5 a.m. local time when a US wire service called asking for reaction to President Reagan's announcment. Major Brooks, totally surprised, immediately called the Department of Defense to ask for direction, but the appropriate official in Washington had gone home. So he then tried European command headquarters in Stuttgart, West Germany, for advice, but it had nothing until halfway through the conversation.
Two other contingents in the four-nation force were unofficially notified Tuesday. The British contingent had completely evacuated by 8 a.m. Wednesday, lock, stock, and bullets, leaving their headquarters hauntingly deserted.
Indeed, the marines were told nothing officially. ''We still have no new orders as to a change in our mission or a change in our status,'' the Marine commander, Gen. James Joy, said Wednesday.
So life appeared to go on as normal. Echo Company, on the northern perimeter, continued to build new bunkers, while other units filled new sandbags, and there was no sign of packing except for the Marine television station that aired its last broadcast Tuesday night.
It was a stunning contrast to the mood in Beirut, one basically of despair, a sign to the Lebanese that the US had given up on Lebanon, that the factions would now be left to battle it out for themselves.
The continued presence of special envoy Donald Rumsfeld and 130 US trainers of the Lebanese Army offered no consolation, for the public sentiment was that they too would soon give up, a suspicion further supported by the evacuation of 41 US diplomats and their dependents on Tuesday, and 49 more on Wednesday.
A US military adviser, Col. Al Baker, tried to reassure the Lebanese: ''The US is not abandoning Lebanon. The Marines are not withdrawing immediately. The noncombatants have withdrawn. But the combatants are still in their foxholes. They intend to stay until this thing is resolved.''
Major Brooks tried to refute public sentiment that the withdrawal signaled defeat. ''We can keep our heads up high and be proud. We have given this country 17 months to try to put Lebanon together again. We feel we've helped the people of Lebanon.''
But others conceded that, in effect, the plug was being pulled. Capt. Hank Donigan, commander of Echo Company, who was also here with the first Marine unit in 1982, said: ''I don't want to have to leave without feeling our mission is complete. The government is in bad straits, at best. I just sorta feel sad, as we were hopeful for these people and this country. Now we can only expect more bloodshed and factional fighting.''
He also said the Marines still had a role to play. ''We don't have a Dunkirk here. I could march my company forward if necessary. We still have the initiative. I think we could hold this crowd a long time.''
But Lance Cpl. Matthew Montanino reflected the thinking of many marines. ''After the other night (of heavy shelling), it's about time they did something. I'm glad I'm going home.''
There was still no official word from the French or Italian contingents about their plans. But as one of their officers noted: ''It is always bad to leave when the situation is bad. But now there seems no choice. The Americans have given us no other option except to leave once they move out.''
Meanwhile, Wednesday's events have heightened tension in the capital. Checkpoints run now by Amal, the Shiite Muslim militia, and Lebanese Army defectors scrutinized documents of all travelers.