President Carter formally opened the butter versus guns debate yesterday by sending Congress a budget calling for spending $125.8 billion on defense in fiscal 1980, a 3 percent increase over the current fiscal year after allowing for inflation.

The rise in military spending stands out like a lightning rod from the generally flat plane of funding proposed for other government programs, a sure attraction for political attack.

Defense Secretary Harold Brown, at a news briefing at the Pentagon on Saturday, characterized the new defense budget as "very austere" but enough to give the nation a "sufficiently balanced" military program.

There are no big surprises in the Pentagon budget. Rather, the money requested would continue to implement the president's decisions to substitute cruise missiles for new bombers, push ahead with the MX blockbuster land missile, build a Navy of smaller and cheaper ships, and deploy "smart" weapons -- accurate enough to kill in one shot -- to offset the Soviet edge in numbers ranked against this country's allies in the North Atlantic Treaty Organization.

The president had said he would live up to his commitment to NATO partners to increase U.S. military spending by 3 percent after inflation. Forces especially suited to fighting a land war in Europe are generously funded in Carter's budget.

Brown said a big improvement has been achieved under Carter in the ability to move U.S. troops to Europe in a hurry. He said that by 1984 the United States will be able to fly five divisions to Europe within 10 days, compared to just one division now.

Carter's budget also shows he has concluded, as did his two Republican predecessors in the White House, that the best defense is a good offense. An extensive civil defense program, championed by some staffers on the National Security Council, failed to pass muster.

The president approved only $110 million for civil defense in fiscal 1980 -- about the current level of $98.3 million for fiscal 1979, when inflation is taken into account.

Strategic forces, which would be used to fight an all-out nuclear war, are slated to receive $10.8 billion in the president's new defense budget, while general purpose forces for nonnuclear conflicts would get $50 billion, or almost five times as much.

The new defense budget goes to Congress at a time when Carter is trying to assure lawmakers that signing a new arms control treaty -- called SALT II -- would be an acceptable risk, especially since the United States will keep improving its strategic offense.

Improvements in the strategic arsenal to be financed under the new defense budget include the following:

MX blocbuster ICBM. The president is asking for $670 million in research money to put the intercontinental ballistic missile into "full-scale development," the last step before production. Additional money for the missle is included in the separate request for $2.2 billion in fiscal 1979 defense funds the president is submitting to Congress. The MX would be a mobile missile to replace the 1,000 Minuteman land missiles that stand still in underground silos. They are becoming vulnerable to increasingly accurate Soviet warheads, according to the Pentagon.

Trident II submarine missile. Carter wants to give the new Trident submarine a longer reach through this missle. He is asking $41 million to speed its development. Critics of Trident II claim the missile would be so accurate that the Soviets would conclude the United States was arming its submarines for a surprise attack rather than retaliatory fire.

Air-launched cruise missile. This version, which Carter chose over the B1 bomber, would go into production in fiscal 1980 with $371 million in procurement money. Other versions of the cruise missle -- the submarine-launched Tomahawk and ground-based tactical weapon -- are being held to a comparatively low level of funding.

One of Carter's most controversial blueprints is for the Navy fleet, with the basic argument one of quality or quantity. Carter has opted for quantity, with his veto of the Nimitz nuclear aircraft carrier last year the most dramatic example.

Instead of the Nimitz, Carter is asking Congress for $1.6 billion to build a conventionally powered aircraft carrier of about 60,000 tons. Several members of Congress have attacked this proposal before and intend to renew the battle this year. If they can't get another Nimitz built, they want a big conventional carrier rather than the small one the president is recommending.

The same quality over quantity argument is expected to swirl around the FFG7 small destroyers Carter wants to continue building. He is requesting $1.26 billion for six more of them in his new budget and money for another one in the fiscal 1979 supplemental budget.

These requests testify to the administration's determination "to build smaller, less expensive ships whenever mission requirements will permit," Brown said.

Altogether, Carter is asking Congress for $6.2 billion to buy ships in fiscal 1980 compared to $4.6 billion in fiscal 1979. Brown conceded that this new total still will not produce new ships faster than old ones are being retired, meaning a shrinking naval fleet.

The Trident missile submarine, a giant that is longer than the Washington Monument is high, continues to take a big slice of the Navy's ship-building budget. The president's new budget requests $1.5 billion to build another of these craft.

In discussing aviation policy, Brown said that the administration intends to reduce the variety of aircraft built for the Navy and Marines. This is one reason the Marines' request for additional money for the Harrier AV8B jump-jet was denied, he said.

Under Carter's new budget, the Army would get money for a wide range of armored vehicles, battlefield missiles, guns and helicopters to continue its modernization.

One Army budget item expected to provoke close questioning in Congress is Carter's request for $144.8 million for a long-range version of the Pershing nuclear-tipped battlefield missile deployed in Europe. The money requested for Pershing II is triple the $42 million approved for fiscal 1979.

Critics argue that a missile based on the ground in West Germany able to penetrate deep into communist territory would provoke protests from Soviet leaders. A counterargument is that the Soviets already have deployed their SS20 intermiate range missiles against NATO.

The Pentagon, in preparing its budget, does not include money for nuclear warheads built for military weapons by the Energy Department. This is why White House and Pentagon totals for national defense differ.

Using the Pentagon's detailed figures, here is the picture of past, present and future miltary spending: (TABLE) Fiscal Year(COLUMN)Spending in Billions 1978(COLUMN)$103.0 1979(COLUMN)111.9 1980(COLUMN)122.7 1981(COLUMN)133.7 1982(COLUMN)144.9 1983(COLUMN)155.5 1984(COLUMN)165.7(END TABLE)

In budget authority, which includes money that can be committed during a fiscal year but not spent, the Pentagon is requesting $135.5 billion in fiscal 1980, an increase of 1.7 percent over fiscal 1979 after allowing for inflation.

In spending, the Pentagon's figure of $122.7 billion (not counting Energy Department costs) represents a 3.1 percent increase after allowing for inflation.