Everyone knows how difficult it is to reduce the defense budget. President Reagan doesn't believe that money spent on weapons is real money or should be counted in the national deficit. We have a secretary of defense who has rarely met a weapon he didn't like.

But the blame doesn't stop there. Congress wails about deficits, vows to make cuts, but can never bring itself to kill weapons programs. Congress is against capital punishment when it comes to Pentagon hardware.

The problem is illustrated by what the Senate Armed Services Committee did recently about the Divad (division air defense) gun called the Sgt. York. The committee gave the administration $256 million less than it requested, but still approved $150 million for fiscal 1986 for the gun.

The Divad is one of the Pentagon's major embarrassments. Its mission is to shoot down helicopters and other aircraft over a battlefield, but its range may be less than that of some weapons carried by attacking Soviet choppers. The Pentagon, which never admits defeat even at the most glaring of weapons Dunkirks, is planning to graft a Stinger missile onto the hapless Divad.

The Sgt. York may be crying out for a mercy killing, but Congress can't bring itself to pull the plug.

Congress rationalizes that it is cheaper in the long run to keep funding turkeys because ending them involves the payment of "penalty" or "termination" fees to defense contractors, the exact amount of which is classified information. Knowing what it does about General Dynamics' billing practices, it may be right.

Even with a cut of 3 percent in the $313.7 billion Reagan budget request, 1986's defense spending would be $19 billion over 1985's.

Bad as it is, it is going to get worse. This somber information comes from Gordon Adams of the Center on Budget and Policy Priorities, who for the past five years has studied defense spending. He is the author of "The Iron Triangle," an analysis of the unspoken conspiracy among the Pentagon, defense contractors and Congress to keep the defense budget growing.

Adams has created considerable chagrin in the Pentagon. Last year, he wrote a report called "Nuts and Bolts at the Pentagon: a Spare Parts Catalogue" and at the publication news conference invited a prominent local hardware store magnate, John D. Hechinger, as an expert witness. Hechinger said the Pentagon could have bought an Allen wrench for 25 cents at his store. The Pentagon managed to lay out $9,600 for one.

Cost overruns, which come from a systematic underestimation of the price of elaborate weapons systems, are an old story. The "backlog" of appropriated but unspent or unobligated funds is another -- the backlog is estimated to rise from $92 billion in 1980 to $280 billion in 1986. Yet, presidents get 95 percent of what they seek for the Pentagon.

Since taking office, Reagan has launched an unprecedented peacetime military buildup, with no end in sight. And his is what Adams calls "a weapons-driven budget." Fifty percent of the 1986 budget will go for weapons. Funds for operation and maintenance, which used to be 32 percent, have dropped to 25 percent.

The money for all that hardware is virtually locked in for years to come.

The situation is such that a new president, even say one of the George McGovern school of defense spending, couldn't do much about it. Adams estimates that by 1989, 85 percent of the the defense budget will be untouchable.

The effect on social spending is predictable. Cruel choices will become more cruel. We face more hideous economies, victimizing the least fortunate.

The president wants to take away free mailing privileges from the blind, something they have enjoyed for 81 years. He proposes, in the interests of cutting the deficit, to take the $38 million cost of sending large Braille books through the mail out of the budget and turn it over to the U.S. Postal Service, which would have to deal with it by increasing postal rates again. The sum involved is less than a quarter of the consolation money bestowed on the worthless Sgt. York, but such are the priorities when weapons matter most.

Pentagon spending has long been out of control. Adams' point is that it is also uncontrollable. And why does the public put up with it?

Adams thinks it's because the ordinary citizen, once he gets beyond the $9,600 Allen wrench and the $718 pliers, feels cowed by the hi-tech jargon of the Pentagon priests, is silenced by the incantation of the magic words "national security" and, like Congress, is often moved by self-interest. Defense contracts, however extravagant, mean jobs and money. The most preposterous weapons acquire constituencies, as the momentum of "Star Wars" attests.

Maybe Pogo's dictum applies: "We have met the enemy and he is us."