Offense vs. defense. For nearly 40 years - since the dawn of the atomic age - offense has dominated military strategy. Nuclear-armed bombers and missiles can easily overwhelm the best defenses.
President Reagan, looking beyond the current ''balance of terror,'' now wants to renew the emphasis on defense.
Leading scientists say that Mr. Reagan's proposal for more research into antimissile defenses could eventually tip the balance back in favor of those trying to defend their homeland from an atomic assault.
The Reagan concept is already triggering debate. Here are some of the immediate questions his plan raises, along with answers from leading United States scientists and planners:
* Is an antimissile defense possible?
No one knows.
In the near term, the answer is ''no.'' Currently, there is only one method to stop an incoming nuclear warhead. That is with a hypersonic missile armed with its own atomic warhead. The missile, using radar and computers, flies close enough to an incoming warhead to destroy it.
Such a system, however, is easily overwhelmed by simply firing more enemy warheads at it. More offense is cheaper than more defense, so the system can be defeated.
Long-term, however, the answer is less certain. Scientists are looking into a number of options, including such exotic weapons as lasers, particle beams, and microwaves. Lasers, for example, might be fired either from satellites in space, or from ground-based stations, to destroy incoming missiles.
* How far are we from developing some of these exotic systems?
Probably more than a decade. Various scientists say that by quickening the pace of research, the US could score a breakthrough. At the moment, however, they don't see a workable system before the year 2000.
* Is the Reagan proposal, then, ''just politics''?
Motives are difficult to judge. However, Albert Carnesale, a Harvard specialist in defense matters, says the Reagan proposal probably comes at about the right time.
For years, spending on defense against missile attack has either held steady or declined. Recently it's gone up a little - to about $1 billion a year. But missile defense has gotten little emphasis. Yet, says Dr. Carnesale:
''The idea of relying forever on deterrence [by mutual destruction] is not good.''
* Could an antimissile system make the world more dangerous?
Maybe, and maybe not. Richard Betts, a foreign policy specialist at the Brookings Institution, says it depends on the scenario.
If only the US were developing such a system, for instance, the Soviets might suddenly wake up one morning to find that their missiles were ineffective. That would mean the US could launch a nuclear strike against the Soviets without fear of retaliation.
If the US were in the process of installing an effective system, Mr. Betts observes, the Soviets might feel they were faced with a ''now or never'' situation and be prompted into a preemptive strike.
However, if each side installed antimissile systems under a carefully drawn timetable, the effect might be stabilizing, experts say.
* What are the Soviets doing?
In the Moscow area, the Soviets operate the new Pushkino Antiballistic Missile Radar, which at present can guide 32 missiles (that will eventually be raised to 100) to destroy incoming warheads. The system is giving the Soviets some useful experience, US scientists say.
The Soviets ABM system, however, could be easily overwhelmed by US missiles. It would be effective only against an accidental US launch, or against smaller missile systems such as the British or Chinese.
Beyond that, the Soviets are deep into research on particle beam and other exotic weapons. Their progress is uncertain.
* Are there moral questions about antimissile systems?
Moral arguments are made on both sides. Some feel a US antimissile system could make the Soviets feel threatened. Others say it would be a positive step to put the US beyond the threat of nuclear weapons. It would, as one physicist noted, bring the US beyond the current world of Dr. Strangelove into the world of Buck Rogers.