FOR SPAIN, last Thursday was a historic day. NATO's 15 members formally invited it to join. The invitation has a strategic aspect: the Atlantic Alliance should be the firmer for incorporating Spain's military assets directly. The principal aspect, however, lies in its domestic significance. The Franco dictatorship had isolated Spain--a personal as well as political burden to Spanish democrats. What international legitimacy the country did enjoy came, after 1953, chiefly from its bilateral military connection with the United States. Now a nation that in its dominant cultural strain feels itself to be a founding father of modern Europe is returning, institutionally, to its roots.

One unfortunate byproduct of the country's long estrangement from the democracies is that, within Spain, public support for entry to NATO does not appear strong. A recent Gallup poll found 27 percent of the people in favor, 39 percent against and the rest undecided. (How would Americans vote if they were deciding now on NATO membership for the first time?) A misplaced nostalgia for the benefits of sideline-sitting has provided a certain soil for Socialist Party appeals to neutralism and pacifism and for Communist Party exhortations along the Moscow party line. The result has been to make the government reluctant to test popular support for NATO entry in a referendum, and to try to move the issue as far along as possible before the next parliamentary elections in the spring. It's a bit embarrassing. It is, however, the duly elected government of Spain that is knocking on Europe's door, and that is plenty of reason to accept the bid as authentic.

There are diplomatic details to see to. Spain is pondering how entry into NATO might strengthen its hand in dealing with the British on Gibraltar and in maintaining its colonial enclaves. The United States hopes that Spain's policy of ruffling no Arab feathers can be combined with access by the new Rapid Deployment Force to some of those old Spanish bases. The important consideration, however, is that with the invitation from NATO in hand and with a second invitation from the European Economic Community expected in a few years, Spain is finally coming "home."