Washington — The Reagan administration's acquiescence on Pentagon budget cuts will not mean immediate obvious changes in defense posture or weaponry. Troops will not be pulled back from Western Europe or South Korea. Nor will big-ticket programs like the B-1 bomber or MX missile be killed.
It's more a question of shape rather than size. The Republican deficit-reduction package gets to the heart of military financing and could have important longer-range effects impinging on national security.
Specifically, the way in which the rate of increase in the defense budget is likely to be reduced (nobody is talking about real cuts here) relates to two important issues: unplanned increases in weapons procurement costs that historically have plagued such purchases; and how ready United States forces are to operate those new weapons and do battle, should deterrence fail.
In essence, the administration has told Congress it now wants to do just as much militarily, but with a smaller increase than it originally sought: 7.5 percent instead of 13 percent for fiscal year 1985, or about $14 billion less than the earlier $305 billion request.
This parallels similar action taken by Congress over the past two years. Even though Congress has pared back Reagan defense budget requests, virtually every new major weapon system has been approved (chemical weapons are the one exception), and reductions have been found elsewhere. Some of the reductions have come from positive adjustments in inflation projections due to the economic upturn. Some have come from stretching out certain weapons programs. And some have come from squeezing certain ''readiness'' accounts.
These are the kinds of things that result in more immediate effects on federal deficits and are therefore more politically attractive. And the administration proposal to lower defense spending by $57 billion over three years does just this. It envisions relatively more short-term savings than the Senate Budget Committee chairman, Pete V. Domenici (R) of New Mexico, proposed in an even larger three-year reduction of $81 billion.
This could put pressure on operations and maintenance accounts (including spare parts and ammunition), critics fear, just when the Pentagon has been facing increased criticism over the combat readiness of forces.
With weapons procurement taking a relatively larger chunk of defense spending since President Reagan took office, the question of cost overruns becomes important as well. Defense Secretary Caspar W. Weinberger maintains that through administration changes to defense procurement - using realistic inflation estimates, multiyear purchasing, and other means - the historical cost overrun pattern will not continue.
But the General Accounting Office recently reported that ''management reforms , too, can suffer from optimistic assumptions,'' and that the problem of cost overruns - which averaged 32 percent over the 20-year period through 1983 - may not have been overcome.
If true, this could mean that reductions in projected defense spending without cutting back some major weapons programs could exacerbate potential problems in readiness of and support for military forces.
Pentagon officials now are combing their defense request for ways to reduce it without bringing on these harmful side effects. They say it can be done, and there traditionally is a bit of ''cut insurance'' built into controversial federal budgets.
On Capitol Hill, Democrats are crafting their own deficit reduction package, which undoubtedly will include defense cuts below what the White House and congressional Republican leaders have agreed upon.
''I believe further reductions in the rate of growth of military expenditures will happen,'' House majority leader Jim Wright (D) of Texas told reporters over breakfast Friday. Congress last year approved an increase in Pentagon budget authority of less than 4 percent, and many experts say a sustained growth rate of 5 percent is better than cycles of big increases followed by sharp cuts.
Administration give on defense spending as reflected in the agreement among top Republicans and the White House last week also takes into account last year's experience and demonstrates a recognition of political realities.
A year ago, the administration at one point might well have been able to obtain a negotiated 7.5 percent increase in defense spending. Instead, it held out for much more and eventually wound up with much less.
This latest White House offer and GOP leadership unity may settle the question sooner. Its eventual impact on national security will take longer to determine.