The Supreme Court yesterday nullified a ruling denying compensation to a soldier whose body was severed by a tank after the Army had recruited him with an assurance of nonhazardous duty.

The justices told the U.S. Court of Claims to reconsider its decision that the Army didn't breach the agreement with which it recruited Allen A. Jackson. Only Justice William H. Rehnquist would have let the ruling stand.

In what Jackson's lawyer termed an "amazing" Medical Feat, the Army saved the soldier's life after a tank ran over him during manuevers at Ft. Riley, Kan., 3 1/2 years ago.

The tank cut Jackson in half below the naval; only his kidneys and bladder remain. Now 24, he lives with his parents in Franklin Park, Ill., near Chicago. He moves about in a prosthetic device, his lawyer, James N. Garlock of San Francisco, told a reporter.

The Supreme Court action accorded with the position of the government, which said that a three-judge panel of the Court of Claims hadn't gotten a key fact straight when it dismissed Jackson's $5 million claim for damages with a statement that it "deeply sympathizes" with him but was powerless to grant relief.

Jackson enlisted in January, 1974, under a written agreement that the Army would assign him after basic training to an advanced, noncombat school for vehicle maintenance at Ft. Riley. His hope was to operate a service station or auto-repair shop after completing a three-year hitch.

But on his arrival at Ft. Riley on May 11, 1974, the Army assigned him to a tank unit. He protested the same day to an officer who acknowledged the assignment was improper.

Nine days later, Jackson had an appointment with another officer on his request for transfer to the maintenance school. But the tank-unit commander canceled the appointment because it would have interferred with manuevers.

Jackson carried out his mission: running 800 yards in front of four tanks and exploding smoke grenades at a certain point in the road. On reaching the grenades, the tanks turned in his direction. One ran over him.

In the Court of Claims, Garlock contended tyhat the Army had breached an enforceable contract.

Rejecting the contention last March, Judge Byron G. Skelton cited language in the enlistment papers in which Jackson agreed to complete "basic or basic combat training (BCT)" before he could attend the advanced school. The next sentence, however, spoke only of completing "BCT."

In an opinion joined by Judges Robert L. Kunzig and Marion T. Bennett, Skelton said it was "clear" that Jackson, having been at Ft. Riley, only nine days, hadn't completed BCT. Actually, BCT and basic training are the same thing.