In September, Texas Gov. George W. Bush laid out his initial vision for U.S. defense policy. His speech contained a sound proposal to increase spending on military research and development. The governor also advocated deploying strategic missile defenses, further increasing military pay and rethinking the desirability of some overseas military deployments.

One overarching problem complicates Mr. Bush's proposals, however: He cannot begin to pay for them. His support for tax cuts is at direct odds with his Pentagon plan. Yet while Bill Bradley and Al Gore constantly challenge the affordability of one another's agendas, Bush's ideas have not yet been subject to similar review.

Congress and President Clinton have demonstrated how hard it can be to reduce funding for federal programs by just one percent. Bush's problem is more difficult. His defense vision appears to cost almost 10 percent more than the resources he would actually provide the Pentagon.

U.S. defense spending is now about $280 billion a year. Under the Republican Congress's tax-cut agenda, which Mr. Bush supports, real defense spending would more or less remain constant over the next decade (actually it would increase modestly in the next few years but then decline even more thereafter). To see why that conflicts with the governor's defense plan, it is necessary to do some simple defense spending arithmetic.

There are basically five main pieces of the U.S. defense budget. Four are pretty much holding steady in cost. The remaining category, procurement, is about to go through the roof.

* "Procurement" of military hardware. The United States is now spending about $50 billion a year to buy military equipment. Under current Pentagon plans, that number will soon have to increase to some $80 billion, as recent Congressional Budget Office studies show, for two reasons.

First, the Pentagon has kept its Cold War habit of buying ever-more expensive weaponry. Second, it needs to replace hardware bought largely during the Carter and Reagan years that is beginning to wear out.

Bush cannot change the second situation. As for the first, he did suggest that the military services "skip a generation" of weaponry. But he provided no concrete suggestions in his speech about how to do so. That vagueness suggests that if elected, he would not expend the political capital necessary to force the services, Congress and the U.S. defense industry to cancel programs they hold dear.

* Operations and maintenance. This catch-all account now costs the Pentagon $100 billion a year, covering the salaries of civilian defense employees, health care, environmental cleanup at bases, overseas military operations, combat training and purchases of spare parts.

The Pentagon can save some money in this account by closing more bases and privatizing some support activities. That might reduce annual costs by $5 billion. If Bush can really streamline U.S. involvement in places such as the Balkans, he might save another $3 billion.

* Military personnel. This category funds salaries plus pensions for uniformed troops and costs just over $70 billion a year. Given the recent decision to give military personnel much-deserved pay raises, its inflation-adjusted level will increase modestly in the future. It might be possible to prevent that increase, perhaps by adopting a less demanding type of "two-war strategy" and reducing troop numbers slightly. But Bush has not advocated such a change. In fact, his call for another pay raise would increase costs further.

* Research and development. The Pentagon spends about $35 billion annually researching, developing and testing weaponry. Bush would add $4 billion a year to the Pentagon's plan--not a bad idea. Among other things, it could help the Army develop the light but lethal armored forces that it knows it needs but cannot yet build.

* Miscellaneous. Four other accounts add just over $20 billion to the U.S. defense budget. They include family housing and military construction within the Department of Defense, as well as nuclear waste cleanup and nuclear weapons stewardship conducted by the Department of Energy. Bush has not proposed cutting these expenses, and it is doubtful he could reduce them much.

Added up, annual defense spending would have to increase perhaps $25 billion, above and beyond inflation, under Bush's plan. That is $25 billion a year more than recent Republican tax-cut proposals would provide the military. The governor has to decide what he wants more, tax cuts or such a defense buildup, because he probably can't have both.

The writer is a senior fellow at the Brookings Institution and adjunct professor at Columbia and Georgetown universities.