THE NEW REAGAN defense paper is not so much a budget as a manifesto. Politically, it announces the administration's determination to show that the American political system can carry through expensive new military programs over a protracted period, notwithstanding the other strains on the country's resources. Geopolitically, it advertises a determination to project American power around the globe -- to newly critical areas like the Persian Gulf and, for that matter, to the shores of the Soviet Union.

The budget is based on the familiar Reagan contention that the United States has failed to meet the Soviet buildup of recent years and is therefore not in a position to cope with the various probes and adventures that buildup has made possible. It is not for this administration to be more selective about the international responsibilities that the United States should be ready to bear. Rather, its choice is to show with greater certainty that it can bear them all.

This budget says that little or no part of American security, broadly conceived, should rest on exceptation of restraint by or cooperation with the Soviet Union. The hope is that, in practice, this approach will produce more Soviet restraint and cooperation than might otherwise be forthcoming. The military contingencies forseen are considerable, and the five-year Reagan defense plan would let the administration meet virtually all of them.

There will be time, in the budget process, for the program choices of the Reagan Pentagon to be examined. What most needs attention now is the bold and ambitious charter of the budget and especially is emphasis on improving the worldwide mobility and visibility of American forces and on building up the Navy. It may help to consider two particular parties or people, among many, who will be reading the budget in their fashions.

The first would be the Poliburo, perhaps Leonid Brezhnev, himself newly reconfirmed with literally all his old colleagues in the Soviet leadership. Will the Politburo be sobered by the spectacle of America mobilizing or provoked to match or end-run the American effort, or pleased at what it may see as the prospect of American overextension? The second would be some unnamed guerrilla currently pushing his way through the mountain country of El Salvador. Will he, too, be intimidated by the bulletin his transistor radio brings him, or will he figure he should move fast to beat the buildup, or will he conclude that it confirms everything he ever thought about the inability of the North American Colossus to deal with revolutionary situations around the globe?

Every budget -- every defense budget -- is a wager on history, but these are some of the central questions that must now be asked.