Why Spain changed tactics on Gibraltar

The Dec. 14 reopening of the frontier between Spain and Gibraltar, closed by the Spanish in l969 to pressure Britain to cede sovereignty of the Rock, is one of the first foreign policy initiatives by the new Socialist government of Felipe Gonzalez.

The gates were opened only to pedestrians and limited to holders of Spanish passports and Gibraltarians with British passports.

The Socialists have consistently said Francisco Franco's closing of the frontier was a mistake, preventing contact between Gibraltarians and Spaniards, and forcing Gibraltar to be more pro-British.

Gibraltar has been under effective seige. Its only contact with the outside world has been limited flights to Britain and Morocco's Tangiers, and a sea-link to Tangiers across the Strait of Gibraltar.

On the Spanish side of the frontier, the closure led to massive unemployment. Many Spaniards from La Linea used to work in Gibraltar's naval dockyards, and La Linea itself was the main recreational center for British servicemen based in Gibraltar.

The opening up of Gibraltar immediately puts at risk the precarious prosperity of Spain's two North African enclaves - Ceuta and Melilla.

The Spanish military, traditionally sensitive over any sign of weakness in negotiating over Gibraltar, have kept quiet over the Socialist move.

A Gibraltarian lawyer said this week, ''Felipe Gonzalez has been very astute; he has given none of Spain's negotiating position away, but he has begun to put right what was wrong in the Spanish position.''

Not all Gibraltarians are so enthusiastic over the reopening of the frontier. Preferring life under British rule, they are afraid this is the beginning of the imposition of Spanish sovereignty on the Rock.

But the British government has been consistent in stating that it will do nothing about the future of the colony against the wishes and interests of its inhabitants.