THAT SENSATIONAL Evans and Novak column saying the United States is considering "a new defense strategy that secretly concedes one-third of West Germany to a Soviet invasion rather than seek increased defense spending" poses a couple of puzzles. Why, for instance, would any responsible official leak something that is preposterous by face, at least in our judgment, and that, true or false, can have only mischievous effects on alliance confidence and, conceivably, on Soviet planning? He would do the leaking, we presume, to build up some political steam for "increased defense spending." Detecting the self-serving aspect of a leak, any leak, is a constant challenge facing the consumers of Washington journalism.

But, you may ask, is there not some truth in the column? There's a lot, if what the columnists meant by "conceding" a third of Germany was that, in the initial phase of a full-scale surprise attack, the Warsaw Pact might seize substantial territory before NATO could effectively respond.Otherwise, the column testifies to the seriousness of current adminstration debate over whether the conventional forces of NATO can deter a Warsaw Pact attack, repel such an attack if it comes, and do so without employing nuclear weapons unless absolutely necessary. NATO's strategy always has been and, according to the President, still is to conduct a "forward defense" with an eye to conceding as little territory for as short a time as possible. But the Warsaw Pact buildup of the last few years has raised doubts about whether NATO can still put that strategy into effect.

It is precisely these doubts that underlie the administration's attempts to freshen its military planning, to beef up the alliance's conventional defenses, to negotiate force reductions in Europe and, incidentally, to open an option to deploy the neutron bomb - an offset for Moscow's widely presumed conventional superiority. The discussion on these matters is intense; the column plugged into part of it.

The column has had, however, one good result. It elicited from the President and his chief advisers pointed reaffirmations of the U.S. commitment to its European allies. Their reliance on this country is so great and permanent that, with or without newspaper stories, they will always wonder whether, in the clutch, the Americans will be there. It is a continuing task of American policy to find persuasive ways to say yes.