It is an open question whether the President would launch American missiles once warning radars showed that Soviet missiles were flying toward the United States, according to Defense Secretary Harold Brown.

Although Brown and his predecessors at the Pentagon have said that the United States has built a nuclear force secure enough to absorb a surprise attack and still retaliate massively afterward. Brown said current doctrine does not rule out launching ground-based missiles before they could be destroyed.

"It is not our doctrine to do so," said Brown of launching U.S. intercontinental ballistic missiles before Soviet ones exploded over the United States. "Neither is it our doctrine that under no circumstances would be ever do so."

A "launch on warning" strategy, arms control specialists have argued, would put an hair-trigger on nuclear warfare. Either the United States or the Soviet Union, they contend, might misread the warning signals and start a nuclear war in response to a false alarm.

On the other hand, if the President did nothing until after Soviet missiles exploded over the United States, he might lose a large portion of the nation's land-based intercontinental ballistic missiles and communications with other strategiic forces. The growing accuracy of U.S. and Soviet missiles is making fixed missiles and communications increasingly vulnerable to destruction.

"I think the strategy encompassed by the phrase 'launch on warning' is not the appropriate thing for the United States to do," Brown said in a closed hearing of the Defense Appropriations Subcommittee on Sept. 15. "I think the way to describe it is, 'launch under attack.' And the question is: would you launch land-based missiles before explosion of nuclear weapons on the United States."

"That is something that should be considered only with the greatest caution," Brown told the subcommittee. "I see no reason to launch submarine-launched ballistic missiles that way, although the bombers ought to be launched on warning because that does not constitute a decision to send them to the target. Each system in the triad" of submarine missiles, land missiles and nuclear-laden bombers "is different."

"Are you indicating," pressed Subcommittee Chairman George H. Mahon (D-Tex.), "that we would not actually launch our ICBMs until after we had been attacked and Soviet missiles impacted the United States?"

"I am not answering that question one way or the other, Mr. Chairman," Brown replied. He added that current nuclear doctrine does not foreclase either absorbing a first strike before launching a retaliatory attack nor firing off American land missiles before they could be destroyed.