For NATO defense planners the British operations in the Falkland Islands provide some good news and some bad news.

The good news is that the war has demonstrated that the United States has a potent ally. The bad news is that the war's continuation could injure relations within the alliance and between the alliance and Latin America. News Analysis News Analysis

Frequently overlooked in the gloomy balance-of-power assessments that come out of Washington and other western capitals every year is the point that, although the West is outgunned and outnumbered by the Soviet-led Warsaw Pact, the allies are good, militarily innovative and equipped with weapons that seem to work well.

The dry and static comparisons that make up the East-West balance sheets never convey the military difference between the United States' having Great Britain--or France or West Germany--as an ally, as opposed to the Soviets' reliance on, for example, the Polish army.

Although the British have lost four warships and some other vessels in the Falklands, their campaign 8,000 miles from home has won the praise of many American defense specialists.

"You have to pay tribute to the superb performance of British arms," Defense Secretary Caspar W. Weinberger told an audience at the U.S. Army War College Thursday.

"We've always known that the Royal Navy and Marines were good, so that's no surprise," a top U.S. Navy official added. But it is still reassuring to see them actually operate and know that their weapons work, he says.

The rest of the good news for NATO, as defense planners here see it, involves an expectation that events in the Falklands will halt a decline in conventional British military power.

For example, U.S. officials are hoping that London will now back away from significant cuts in the Royal Marines and in the force of about 50 destroyers and frigates that were planned before the Falklands crisis. Similarly, they expect that Britain will cancel plans to sell one of its two aircraft carriers to Australia.

New emphasis on conventional forces, however, could cause upheaval in Britain's $15 billion plan to replace its aging fleet of four Polaris missile-firing submarines with four new vessels and the U.S.-built Trident II missile.

The plan to modernize Britain's nuclear deterrent against the Soviet Union with Trident has been controversial in Britain since it was announced in 1980, and it may become even more controversial.

How much bad news the Falkland Islands operations will mean for NATO remains to be seen, but the potential troubles are political and military.

Politically, the crisis has driven a serious and totally unexpected wedge between the United States and much of Latin America and between much of NATO, which has supported Britain with economic sanctions against Argentina, and Latin nations.

It has also caused internal disruption in the alliance, especially between Italy, which has refused to continue the formal ban on Argentine imports, and Britain.

The duration of the NATO-Latin American split and the extent of its diplomatic and political damage are key questions. The answers seem to depend on how the undeclared war ends and, assuming a British victory, what Britain will do when it retakes the islands. The same questions are at the heart of some of the key military concerns that could spell bad news for NATO.

Other than the U.S. fleet, the Royal Navy is NATO's biggest and best. It is responsible for about 70 percent of NATO's antisubmarine patrolling in the North Sea, the English Channel and the ocean gap between Britain and Greenland through which much of the Soviet submarine fleet must move to reach Atlantic shipping lanes.

About half of Britain's 62 major surface warships have been diverted to the Falklands, and at least four frigates and destroyers have been sunk there.

For the time being, U.S. officials are not worried about Britain's distraction from its NATO role. Weinberger calls it "a temporary displacement, not a loss" at this time.

But if Britain stays on the islands indefinitely without a negotiated settlement with Argentina, there could be a large, open-ended displacement of British land, sea and air forces from NATO to an area of virtually no Soviet threat.

This would require other NATO countries to fill the British role in the North Atlantic. The United State is providing extra aerial refueling tankers in Europe to replace British craft sent to the South Atlantic.

"What may be a short-term victory" for the British, Sen. Sam Nunn (D-Ga) said recently, "may end up being a long-term burden." Britain, he said, would have to convoy every ship or plane that goes into the Falklands to protect it from Argentine attack.

Similarly, U.S. officials fear that Argentina will turn to the Soviet Union or Cuba for military aid and provide a surprise opportunity for Soviet influence in the area.

The Falklands fighting has also reopened two crucial questions about naval warfare. One involves the vulnerability of expensive surface ships to cheap but effective missiles fired from aircraft many miles away.

U.S. admirals and Navy civilian leaders argue that the U.S. fleet, with the heavy strike and defensive power of its big aircraft carriers, is much better able to cope with this than the British, operating far from home without benefit of nearby land-based aircraft that can help spot the enemy.

The other question involves extensive use of aluminum in the superstructure of ships in the British and U.S. navies. While this lightens the vessels and adds to stability and speed, aluminum's low melting point relative to steel's also appears to be a major fire hazard. The U.S. Navy recently decided to use steel for two classes of new vessels.