One of the truisms that Gen. Colin L. Powell, chairman of the Joint Chiefs of Staff, likes to repeat in these days of global political change is that the Soviet Union will remain into the future the only nation on Earth that could destroy the United States with its formidable nuclear arensal.

For 40 years, the U.S. insurance policy against a preemptive Soviet nuclear attack has been an equally destructive nuclear arsenal linked with elaborate care into a massive retaliatory plan controlled by the president and known by its acerbic acronym, SIOP, for Single Integrated Operating Plan.

The SIOP presents the president with a range of options to respond to anything from a single nuclear warhead strike by the Soviet Union to all-out thermonuclear war. According to one former strategic planner, it includes about 21,000 targets in the Soviet Union among which U.S. strategic commanders could distribute the roughly 11,000 warheads in the American arsenal.

Many in Congress and some Pentagon officials are beginning to plan for a world in which nuclear war with the Soviet Union and the SIOP are less immediate concerns than they have been for the last generation. But a group of Air Force planners is proceeding on the assumption that the coming decade should be a period of growth for nuclear war planning.

Deep within the Air Force's classified spending plans for 1992-97 is a memorandum that outlines the services' $1 billion five-year budget and upgrade of the Strategic War Planning System, the vast computer network responsible for maintaining the SIOP as a constantly updated blueprint for launching missiles, feeding them target data and sending manned bombers on predetermined courses.

A four-page memorandum on the system describes it as "an end-to-end war planning process" serving the president's "mandated guidance for nuclear and conventional missions."

"This process plans, disseminates and implements strategic war plans at the force and unit levels {and} provides the vital link between planners and the weapons systems," it says.

Some startling statistics emerge on the sheer scale of the computer brain for the American nuclear war apparatus headquartered at Offutt Air Force Base in Omaha, Neb.

Describing the "Triad Computer System" and the "Strategic Mission Data Preparation System," the Air Force memorandum says, "The system consists of four main frames, 500-plus terminals/workstations, 21,000 {data} tapes and associated equipment."

"Software to support force level planners producing the SIOP, presently includes over 21 million lines of code," the memo said. A line of code is a single set of instructions telling a computer how to perform computing operations in sequence.

By comparison, the new generation of U.S fighters, bombers and submarines run on computers that carry one million to three million lines of software code.

The Air Force memo says the SIOP "software investment is estimated to be over $1 billion."

The current SIOP computer system was designed in the late 1970s and early 1980s and is based on the IBM 370 mainframe computer. But even with steady budget increases during the defense buildup of the 1980s, the SIOP computer today is overloaded, the Air Force asserts.

Notes the memo, "The storage capacity for the Triad Computer System is already filled to capacity during the peak SIOP production periods and must be increased to meet the rapid requirements increase."

Among those requirements increases are new weapons systems the Air Force wants to add to the strategic triad, such as the B-2 "stealth" bomber and the Midgetman single-warhead missile.

After new features are added in fiscal years 1993 and 1994, the SIOP computer system will be ready for the 21st century and its mission to guide, if necessary, U.S. strategic forces in coordination "with NATO forces to ensure optimum utilization of weapons and systems."