SECRETARY of Defense Richard Cheney and Joint Chiefs Chairman Gen. Colin Powell have little patience with those who call for deeper cuts in the Pentagon budget now that the Soviet Union has virtually disintegrated. The budget already is in a planned decline, they assert. And they have a point. A 25 percent reduction in forces over the next five years represents an about-face from the Reagan military buildup.Still, calls for even deeper cuts have the momentum of events: (1) the disappearance of an expansionist Soviet ideology that threatened Europe and provided a rationale for much US defense spending and (2) the continued upward spiral of US budget deficit projections. The Congressional Budget Office projection for 1995, for example, is now $157 billion. Last winter it was $69 billion. Mr. Cheney, blitzing the morning talk shows recently, reminded viewers that the Soviet inventory still includes 30,000 nuclear warheads. True, but those warheads are no longer in the hands of men obsessed with maintaining Soviet "superpower status." The leaders of the now- sovereign republics care little about such things and will probably lean toward much deeper cuts in nuclear forces than those envisioned by the recently signed START pact. Will the US be ready to take them up on that? General Powell, visiting the Monitor last week, affirmed that reductions already in the works - one-third of the Army's active forces and one-third of the Navy's fleet, for example - represent a "base force," the minimum needed. But tucked into the base-force budget are a number of big-ticket items - like the B-2 bomber or the Sea Wolf attack submarine - specifically designed to counter Soviet threats. They deserve close scrutiny and probable excision. The goal now, as before the coup, is US forces geared to a changed but still turbulent world. General Powell rightly quotes Clausewitz: "Beware the vividness of transient events." US armed forces shouldn't be reduced precipitously, but in an orderly and efficient manner - unimpeded by arguments that often have about as much currency as Lenin's political theories.