The aircraft carrier Nimitz was ordered to forgo its planned port call in Italy and join other U.S. forces being assembled near Lebanon yesterday to provide President Reagan with a big stick if he wanted to use it in connection with the hijacking of Trans World Airlines Flight 847.

But administration officials, in confirming the movement of the Nimitz, the guided-missile destroyer Kidd and other ships to the trouble zone, stressed that the same military realities that have kept U.S. power leashed during previous terrorist actions apply to the current situation.

The first reality, they said, is that ordering specially trained anti-hijacking troops like those of the Delta force at Fort Bragg, N.C., to storm the airliner might cost more lives than it would save, partly because they probably could not overcome the dozen or so hijackers before passengers were shot.

U.S. officials acknowledged there were contingency plans to use specially trained troops to foil hijackings but refused to comment on reports that the Delta force had been flown to Cyprus. The French news agency Agence France-Presse said 15 U.S. soldiers left the Cypriot airport at Larnaca for Beirut at 4:30 p.m. Cyprus time yesterday.

The second reality, U.S. officials said, is that U.S. troops cannot be visible anywhere near the hijacked airliner for fear of derailing negotiations with the hijackers inside the plane or causing them to panic, perhaps prompting them to shoot passengers. Infiltrating American troops to the plane in disguise, as maintenance workers for example, would be difficult at an Arab airport because of their sharp differences in appearance from the regular work crews.

After this TWA airliner hijacking has run its course, other military realities will come into play when U.S. policy makers discuss the pros and cons of retaliatory action. These realities have inhibited President Reagan previously from retaliating against terrorist acts against Americans in Lebanon, despite his public vows to do so.

The most recent example came last fall came when A6E bomber crews on the aircraft carrier Eisenhower rehearsed, while in the Mediterranean, bombing raids against the Lebanese city of Baalbek near the Syrian border, officials said. The idea was for the carrier to stay several hundred miles west of Lebanon for safety and surprise and rely on the bombers' long range, plus refueling by aerial tankers, to strike Baalbek at night.

That retaliatory raid was called off, as were several others planned and almost executed against Lebanon from carriers in the Mediterranean, partly for military reasons that still apply, U.S. officials said. The identity and location of the organization sponsoring the terrorism could not be identified with certainty; there was no guarantee that American planes would not be lost on the retaliatory raid nor that innocent civilians in the vicinity of the target would not be killed.

U.S. officials said yesterday that they were not sure whether the hijackers were sponsored by a Shiite central command or acting on their own, raising the question of how U.S. military force could be applied directly to the source of this latest terrorism.

If U.S. or Israeli intelligence specialists managed to identify and locate the sponsoring terrorist organization, and Reagan should opt for military retaliation against it, the U.S. would still have to decide what force to use. Battleships' guns firing from off shore have relatively short range and can be inaccurate, as they were when the battleship New Jersey was used for this purpose. Carrier-based bombers are capable of deeper penetration and, if conditions are right, greater accuracy. But U.S. bombers crews could be shot down and killed or captured,as they were the last time Reagan ordered a retaliatory bombing raid in Lebanon, Dec. 4, 1983.

Two bombers were lost, one from the carrier Kennedy and the other from the Independence, in a raid against Syrian targets that many Navy leaders considered too minor to be worth the cost.

However, the Joint Chiefs of Staff have warned Reagan in the past that retaliatory raids to avenge terrorism could bring more of it, perhaps increasing rather than deterring violent acts against Americans around the world.