Washington — At this midpoint in Ronald Reagan's first term as President, the Defense Department has gotten nearly everything it asked for: more planes, more ships, more tanks, and a fatter payroll for men and women in uniform.
But this is likely to be ''the year of reckoning'' for the military establishment, as one Republican congressman puts it. Already, the early weeks of 1983 are filled with signs of disquiet over Pentagon spending:
* Republican Senate leaders Robert Dole and Pete Domenici (chairmen, respectively, of the Finance and Budget Committees) told reporters over the weekend that recent modest administration reductions in proposed Pentagon spending do not go far enough.
* A group of 31 Republican members of Congress recently wrote the White House that ''with increased unemployment, high interest rates, and inflation plaguing our economy, it is difficult to support an across-the-board defense 'wish list' for weapons programs.''
* At his first press conference, Robert Thompson, new chairman of the US Chamber of Commerce, last week urged deeper cuts in defense as part of an overall reduction in federal spending.
* This week, some 500 business and financial leaders are beginning a national advertising campaign calling for major budget reductions, including $25 billion from defense. This group is headed by former Commerce Secretary Peter Peterson, and includes five former Treasury secretaries.
* A new study by the relatively conservative Heritage Foundation is highly critical of the Pentagon's expensive weapons-procurement plan, and says the Reagan administration has failed ''to open itself to the fundamental changes needed to break the continuing cycle of a badly strained force structure, low readiness, and unbalanced force capabilities.''
Like war itself, defense budgets don't fit neatly into spaces of 12 months. Major weapons take years to design, build, and pay for. The course of management and payroll planning for 2 million uniformed personnel and 1 million civilians is as hard to shift as that of the proverbial battleship.
Riding the crest of public support for increased defense spending that accompanied the last presidential election, the Reagan administration front-loaded its military planning with expensive weapons systems. But now - just as that public and political support is waning - the bow wave of bills for those ''big ticket'' items is on the horizon and rapidly approaching Washington's shores.
Rep. William Green (R) of New York points out that (absent congressional cuts) spending for the MX missile will double to nearly $4 billion in the coming fiscal year; outlays for the B-1 bomber will triple, and the bill for building two new nuclear aircraft carriers will almost quadruple.
The Congressional Budget Office notes that defense spending tied to earlier procurement decisions will increase from 30 percent last year to 40 percent by 1987. The CBO also warns that the administration's five-year, $600 billion weapons shopping list may actually cost $48 billion more. Estimates by the Heritage Foundation and others (including some analysts within the Pentagon) put the figure at $100 billion or more.
Thus one historical factor, cost overruns, and a new element, increasing weapons procurements, are converging to produce what some analysts fear will be an armed force with lots of shiny new equipment but not enough personnel and funds to operate it effectively.
''Strategy, tactics, even unit structures, are too often fitted to the supposed benefits of the new equipment, when in fact much of that equipment is neither improved in concept nor particularly useful in battle operations,'' says the new Heritage Foundation report.
Defense Secretary Caspar W. Weinberger recently acceded to political pressure coming through the White House and agreed to reduce Pentagon spending for the next fiscal year by some $8 billion ($11.3 billion with future obligations added in).
Military spending would still go up $30 billion (more than 14 percent) over the current fiscal year, however, and the Weinberger-White House trims do not address what many see as the more important problem of major weapons systems. Instead, they focus on such short-term savings as reduced inflation and fuel costs and a freeze on military pay.