United States troops invaded North Vietnam for the first time last Friday when a small detachment of Army and Air Force volunteers landed 23 miles west of Hanoi in what was said to be an attempted rescue of American prisoners of war.

At a press conference yesterday Defense Secretary Melvin Laird said that the raiders had found the P. O. W. camp vacated and had escaped without casualties.

Laird said that he had recommended the alleged rescue operation-which he indicated the Defense Department had been secretly planning for months-when he learned recently that Americans were dying in the prisoner camps.

'Shown Real U. S. Policy'

Senator J. William Fulbright (D-Ark.), said yesterday that the raid, the renewed bombing of North Vietnam, and the invasion of Cambodia "add up to a pattern... which shows us that the real policy of this administration is military victory, not negotiated settlement."

The American forces landed by helicopter at Song Tay, 23 miles west of Hanoi and found the camp abandoned. Laird said.

Brig. Gen. Leroy J. Manor, one of the raiders' commanders, estimated that the camp had been evacuated as long as three weeks ago. He dismissed the possibility of a security leak.

Although fired upon by North Vietnamese SAM's during the raid, the Americans suffered no serious casualties. Col. Arthur D. Simons, the other commanding officer, said that he believed some North Vietnamese had been killed.

Ronald L. Ziegler, White House Press Secretary, said yesterday that the raid "barely missed." He said he doubted that the raid would bring further harm to U. S. prisoners, but said that "if there were reprisals, the U. S. will hold the leaders of North Vietnam personally responsible."

Speculation in Washington yesterday focused on how the American action might affect the Paris negotiations. Some State Department officials expressed surprise that the Pentagon cited lack of progress at the peace talks as a justification for the raid.

North Vietnam has already announced that its representatives will boycott tomorrow's session of the talks. A military reaction "ten times more powerful" than the raid will be forthcoming, North Vietnamese sources in Paris said last night.

Senator Robert Dole (R-Kan.) said that he supported the military action because it showed that the country was concerned for the fate of its prisoners.

Four-hundred fifty-seven Americans are held prisoners of war in North Vietnam, according to the most recent Pentagon figures. North Vietnamese officials have charged that American P. O. W. camps were bombed by American planes over the weekend.