A SMALL SLIVER of "peace on earth" appeared in the Capitol last week.

Like a lot of things in Washing ton, it came with a big price tag.

The defense-spending portion of the continuing resolution actually came out of a House-Senate conference with a ban on anti- satellite weapons testing.

It is a wonderful thing for people who think that "peace through strength" means more strength than peace.

It is, as its author, Rep. Les AuCoin (D-Ore.) says, "the first arms control achievement in the five years of the Reagan administration."

AuCoin doesn't deny that the price was staggering. It was so high, in fact, that Monday night, his liberal allies in the House rose up and voted down the whole conference report, which contained funds for about half the government.

Except for the A-Sat ban, the Democratic House was mauled by the Republican Senate. The House had voted a defense budget of $292 billion. The Senate wanted $302 billion. The so-called compromise -- $297.4 billion.

Rep. Barney Frank (D-Mass.), on looking at the figures, quipped, "The only way to get arms control around here is to buy it."

The liberals had to be glad that they did manage to tack down $6.3 billion that was floating around in what is known as the "Pentagon's Gramm-Rudman slush fund" -- the unassigned money that Defense keeps under the pillow so that Gramm-Rudman cuts that affect all other departments won't hurt in Fatland.

The House wanted four reforms in military procurement. At the 11th hour it got one: an amendment on "allowable costs" which forbids defense contractors to kennel their dogs or join country clubs on the taxpayers' tab.

Rep. Barbara Boxer (D-Calif.), sponsor of the amendment, said that "the important thing is not that we got one out of four, but that we got anything at all. And we had to shut down the government to do it."

The late-night debate in the House pitted AuCoin and Frank, who are allies, against each other. AuCoin wanted the House to vote for the conference report on the grounds that to reject it might mean the loss of the A-Sat ban, which he felt was worth it all. Frank disagreed, and asked sardonically, "If we have to pay this much for A-Sats, how much for a nuclear freeze? Twenty billion?"

Outrage prevailed, and the House sent the conferees back to glare and shout at each other across the table.

The Senate conferees, hawks all who think that every day should be Christmas at the Pentagon, hung tough. Only when AuCoin and Rep. William Chappell (D-Fla.) persuaded Barry Goldwater that the House would rather close down the federal government than accept a bill bereft of reform did the senators give way.

Nerve gas was another Dunkirk for the doves. Production funds for binary weapons were untouched.

AuCoin and Rep. Tom Downey (D-N.Y.) sent around to their Democratic colleagues an extremely candid memo about what had been won and lost. They put this question: "Should we have sacrificed A-Sat control to gain on nerve gas, spending and procurement reform?"

They answered it this way.

Other issues could be revisited. The train is leaving on A-Sats. The president thinks highly of them, for themselves and what they can do for "Star Wars" development. He had one tested just before the Geneva summit, just to give Gorbachev a whiff of grapeshot, and he lobbied hard against the moratorium.

The day the defense-spending conferees first met, Dec. l2, the Pentagon sent up for A-Sat weapons. They cost $20 million apiece and arms controllers were supposed to quail at the thought of squandering public funds.

The House has often shot down A-Sats, but the Senate always put them back up. The arms control case against them is that they sharpen the danger of nuclear attack. If a nation's eye in outer space is blinded, the threat of a first strike is immeasurably increased.

The Soviets proclaimed an A-Sat moratorium in 1982. The administration makes the familiar argument that they only did it because they were "ahead."

AuCoin thinks that the president was planning another shot just before the second summit with Gorbachev in June.

That can't happen now, unless the president vetoes the whole continuing resolution.

Predictably, Defense Secretary Caspar Weinberger, who makes a fearful clamor if a single bullet is removed from his bulging arsenal, fulminated against the moratorium. He accused its authors of undercutting arms control a curious thought from an administration that has never produced an agreement.

"The shooters are livid," says AuCoin. That should make the rest of us reasonably cheerful in the festive but expensive season.