Presidential Directive No. 59, which Jimmy Carter signed on July 25, revised U.S. nuclear strategy by changing targets in the Soviet Union from cities to military installations, from populated areas to leadership centers.

Carter's decision stirred a storm of protest because, it was said, PD-59 added legitimacy to the concept of a "limited" nuclear war. Both sides, according to the critics, will now be more willing to unsheath their nuclear weapons because it wouldn't necessarily start an all-out holocaust.

In fact, Carter's directive is only the latest refinement of a policy that has been on the books for six years. On Jan. 17, 1974, Richard Nixon affixed his signature to National Security Decision Memorandum No. 242. That document, still stamped "Top Secret-Sensitive," was the parent of PD-59, White House sources told my associate Dale Van Atta. The two papers contain largely the same message.

The Nixon memorandum made the new nuclear strategy appear technical and routine. Indeed, in the introductory words, Nixon himself argued that it was no big deal:

"I have reached the following decisions on the United States policy regarding planning for nuclear weapons employment. These decisions do not constitute a major new departure in U.S. nuclear reflect both existing political and military realities and my desire for a more flexible nuclear posture."

When the existence of PD-59 was made public, Carter adopted the same line. The United States had developed some "technological" refinements that permitted Soviet targets to be pinpointed more accurately, his spokesmen explained.

Nixon's top-secret policy statement outlined three general plans -- one for deterrence, another for limited nuclear war, the third for a general war. "The fundamental mission of U.S. nuclear forces is to deter nuclear war," the then-president wrote, "and plans for the employment of U.S. nuclear forces should support this mission."

Then he warned, nevertheless, that just because the enemy didn't use nuclear weapons, it would "not preclude U.S. use of nuclear weapons in response to conventional aggression."

The "deterrence" plan had three objectives: (1) "to deter nuclear attacks against the United States, its forces, and its bases overseas;" (2) "to deter attacks . . . against U.S. allies and those other nations whose security is deemed important to U.S. interests"; (3) "to inhibit [political and military] coercion of the United States by nuclear powers and . . . help inhibit coercion of U.S. allies by such powers."

These deterrence objectives are essentially the same as those Carter endorsed in PD-59. The second section of the Nixon memorandum calls for developing plans "for limited employment options which enable the United States to conduct selected nuclear operations."

The threat of a surgical nuclear strike, Nixon wrote, could be used "to seek early war termination on terms acceptable to the United States and its allies [and] at the lowest level of conflict feasible." The United States would have to hang tough, he said, to show our "determination to resist aggression," though "coupled with a desire to exercise restraint."

This would mean, Nixon contended, that the United States had to have the potential to "hold some vital enemy targets hostage to subsequent destruction by survivable nuclear forces" and to control "the timing and pace of attack execution, in order to provide the enemy opportunities to reconsider his actions."

Nixon's goal in the event of a general war was "to obtain the best possible outcome for the United States and its allies." This deceptively simple statement was then detailed with three subordinate objectives:

"(1) Maintenance of survivable strategic forces in reserve for protection and coercion during and after major nuclear conflict.

"(2) Destruction of the political, economic and military resources critical to the enemy's postwar power, influence and ability to recover. . . as a major power.

"(3) Limitation of damage to those political, economic and military resources critical to the continued power and influence of the United States and its allies."

After this was proclaimed, a target list of Soviet cities and essential military bases was deposited at Strategic Air Command headquarters near Omaha, Neb. More than half of the several thousand individual U.S. atomic warheads were aimed at Russia's military centers.

Carter's plan shifts a higher percentage of these warheads to military and political targets -- including the underground bunkers in which the Kremlin's high command would seek shelter.

But Jimmy Carter's PD-59 is the baby of Richard Nixon's NSDM-242. Another way of saying it is that the world has managed to survive what is now proclaimed as a "new" nuclear policy for the past six years.

This is at least a small comfort.