THE PRESIDENT has gotten himself into a whole new predicament by his decision on the neutron bomb. He has entered a volatile emotional thicket that has been the scene of Atlantic alliance turmoil for 20 years. Mr. Reagan has also handed Europe's burgeoning left, not to speak of the Soviet Union, a powerful propaganda club to use against the United States, and he has taken a step to overload the European nuclear circuit and diminish chances that the new Pershing II and ground-based cruise missiles will be deployed in Europe.

The neutron bomb is no ordinary nuclear weapon. It is a tactical battlefield weapon designed especially for use in Europe in the event of a Soviet invasion. Many Europeans are upset, as well they might be, to think in those terms. It upsets them further to consider the special physical characteristics of the neutron bomb--it is heavy on radiation as against blast and heat--that make its use feasible on European territory. It is not simply that these characteristics lend themselves to propaganda: Nikita Khrushchev's line of 1961--that a neutron bomb meant "to kill people but preserve all riches" and that it embodies "the bestial ethics of the most aggressive exponents of imperialism"--is still being peddled. It is precisely because the neutron bomb seems so feasible that many Europeans have long feared that it would be used.

The strategic argument for the neutron bomb is summarized on the opposite page today by the secretary of defense. The essential case is that this weapon, being relatively small and controllable, would give NATO a credible nuclear deterrent that, even if employed, would not likely produce a general war. The con side holds that neutron bombs, being practical, would lower the barrier against nuclear usage and lead on to a full exchange.

The special and familiar emotional past of the neutron bomb ensures, however, that the issue will not be resolved on strategic grounds. Even before Mr. Reagan decided to move from building to assembling the parts of this weapon, a strong anti-nuclear current was running in Europe and a strong current of distrust of the Reagan hard line as well. Already, the critical alliance effort to deploy new missiles to counter the Soviet buildup of SS20s was an uphill struggle. Prudence dictated that Mr. Reagan reserve a move on the neutron bomb until he had climbed that hill. Instead, he chose to make it steeper. He may be moving NATO toward a fateful crisis.