SECRETARY OF STATE Haig renewed his denunciations the other day of terrorism by leftist guerrillas in El Salvador. Simultaneously, France and Mexico were extending a sort of semi-recognition to the guerrillas as a "representative political force." Their announcement illustrates an abiding principle of world politics: when U.S. diplomacy be comes assertive, other governments frequently look for public gestures by which to disassociate themselves from it. In this case, the price of the gesture is likely to be paid by the unhappy people caught in the midst of the fighting.

In France, a Socialist president has been elected by an alliance of the traditional left with a lot of voters who simply didn't like his predecessor. That produces a hybrid foreign policy. On the vital questions of European defense and Soviet relations, President Mitterand is firm and unsentimental. But on El Salvador he has taken an inexpensive opportunity to demonstrate to the French that his heart is, after all, with the left. In Mexico, the government is moving toward elections. The style of its self-perpetuating ruling party is to remain well to the right on internal matters, particularly in anything involving economics, but to balance it by a dramatically leftist approach to foreign affairs.

Perhaps it will occur to you that all of this explanation has less to do with the realities of the warfare in El Salvador than with the pressures of domestic politics in other countries. Unfortunately, Secretary Haig invites this kind of a response when he persists in presenting the fighting in stark terms of a struggle of forces of light and darkness. Cuba is demonstrably an important source of weapons to a guerrilla movement. But to cut off the Cuban shipments would probably not, by itself, end the fighting. To overstate the Cuban influence is an error of perspective--similar to, although less gross than, the error of those Europeans who say that the guerrillas are merely good socialists like themselves who believe in democracy, civil liberties and social progress.

The first effect of the Franco-Mexican gesture will be to encourage the guerrillas. The second effect will be to increase the sense of isolation within El Salvador's government, and its dependence on security forces over which its control is already inadequate. The ultimate effect will be to increase the desperate determination on both sides to press for a military solution, regardless of the casualties and the desolation of the country. The present display of international posturing is profoundly unhelpful to any attempt to restore peace under a stable government representing a majority of the people who actually live in El Salvador.