Most Americans will readily agree that it would be foolhardy for the United States to scrimp on its military defenses. At the same time they should be aware that the Pentagon budget appears to be slipping out of control in some major areas and that such unrestrained spending threatens bigger and bigger deficits and could thereby thwart economic recovery. As lawmakers in Washington resume work, they need to take a hard look at President Reagan's military budget with a view to scaling back - not the defense buildup itself, but the rate of buildup over the next five years. Such an adjustment could strengthen the economy and thus national security as well.
To understand the situation, the public needs to know these basic facts:
* The proportion of the budget going for weapons procurement is going up while the share for operations and maintenance - in other words, readiness of the armed forces - is going down. In fiscal 1981, 27 percent of the budget went toward procurement of weapons; in fiscal 1984 the figure is 36 percent - and it is slated to rise to 40 percent in 1988. Many critics warn that, given the complexity of new weapons and the need for maintenance, the US in future could face a terrible crunch for funds. Military readiness could be impaired.
* The defense budget has two aspects: authorizations and outlays. In the Reagan budget the proportion of outlays linked to prior-year ''total obligatory authority'' is growing. With each year, in other words, it will be harder to control defense spending because many new weapons will already have been authorized. In the Reagan budget for fiscal 1984 authorizations comprise 35 percent of the total outlays; by fiscal 1988 it will be 43 percent.
* The Pentagon is underestimating the cost of future weapons. Despite the administration's early efforts to provide realistic cost figures, it seems not to be doing so. An internal report by Pentagon analyst Franklin Spinney (suppressed for a time) states that Mr. Reagan's five-year military plan is based on questionable predictions of declining unit weapon costs. In the ''real world,'' the analyst told a House committee last month, the unit costs of equipment always tended to be higher than estimated. The five-year plan, in other words, will either cost even more than the already astronomical $1.8 trillion estimated by the administration, or the Pentagon will be forced to buy fewer weapons.
Recently, too, Pentagon officials looked for some examples of cost-saving weapons programs which they could cite to Congress to defend their budget projections. They could not come up with a single one.
What is to be done?
Two things. One is to take the congressional shears to some major weapons programs. The political difficulty is that the leg
islators are more inclined to reduce operations and maintenance outlays rather than new weapons programs because the latter can represent jobs in their districts, and because cuts in maintenance costs mean immediate budget savings. Yet it is the weapons programs that are driving the budget upward and shortchanging readiness. Will the lawmakers have the political courage to cut job-creating programs in the interests of the larger economy? And of better military preparedness?
The second thing is related to the first: The US must match military spending to missions - that is, it must shape a realistic strategy for what needs to be done and then procure only those weapons required to fulfill the essential tasks. This is not a namby-pamby call for reducing America's capabilities. Such knowledgeable and experienced public figures as Maxwell Taylor, Elmo Zumwalt Jr. , Robert McNamara, Cyrus Vance, and John Connally believe that the Reagan five-year defense plan can be trimmed substantially without impairing US security. Duplication of weaponry is rife. Does the Air Force, for instance, need B-52 bombers, B-1 bombers, Stealth bombers, and air-launched cruise missiles?
To reiterate, the issue is one of reducing the rate of growth in defense expenditures, not stopping the military modernization. President Reagan calls for a 10 percent annual real increase over the five years. Even trimmed by the $ 8 billion to $10 billion or so which Mr. Reagan says he is willing to cut in fiscal 1984, the program is too ambitious, and, most important, elements of it are unnecessary. Economists point out that cutting even one percentage point of that increase would produce tens of billions of dollars in savings.
It is up to Congress to bite the bullet.