President Reagan has moved swiftly to erase misunderstandings in Europe about a spontaneous remark he made recently about nuclear war. There now should be no ambiguity on either the Europeans' or the Russians' part. No aggressor should believe, the President said, that the use of nuclear weapons in Europe would be limited to Europe. Any military threat to Europe would be regarded ''as a threat to the United States itself.''
This represents no change in the US posture. But it should help dispel European jitters that Washington is willing to let Europe become a nuclear battleground - an impression Mr. Reagan conveyed to some the other day. We believe the Europeans overreacted. Yet there is a lesson in all this for a US administration which has not seemed sensitive to the popular fears seething in allied nations and, strangely, has helped fan them by its own hawkish statements.
Common sense dictates that, at a time when nuclear weapons have reached such technological sophistication - making it possible even to talk of a ''limited nuclear war'' - the more restraint in public pronouncements the better. The danger is that abstract, glib discussions of the subject become unrelated to the real world. As former US Secretary of State Dean Rusk writes, ''Anyone who thinks that an all-out Soviet attack on Western Europe, including the American conventional and nuclear forces stationed there, would not lead to an all-out nuclear war is living in a dream world.''
The President echoes this when he says that in a nuclear war ''all mankind would lose.'' Indeed it would. By following up his effectively simple statement with moves toward genuine nuclear arms negotiations, Mr. Reagan can help restore confidence abroad that the United States is indeed doing everything it can to safeguard the peace.