Pentagon planners, peering into the future, have their eyes on a number of developments in the Soviet Union that could pose serious new challenges.
* Soviet fighter pilots, long hampered by rigid tactics, are getting better. Their training now emphasizes more initiative and independence as a means of challenging American air superiority in Europe.
* Soviet industry, making large investments, is beginning to close the technological gap with the United States. Among the worrisome areas of Soviet gains are nuclear submarines and precision guided munitions.
* Soviet military units are developing a major potential to fight chemical warfare. The USSR currently has 14 factories capable of producing chemical weapons, while the United States has none.
These are only a few of the long-term concerns of US military planners - concerns that could become the major defense stories of tomorrow.
The record $305 billion US military budget proposed Feb. 1 by President Reagan addresses the foremost problems on the defense horizon. It continues to rebuild America's strategic arsenal. It strengthens the nation's conventional military might by producing more tanks, aircraft, and ships.
The higher spending of the Reagan years has gradually restored confidence among US military planners. The outlook for deterring war, says Gen. John W. Vessey Jr., chairman of the Joint Chiefs of Staff, is better today than it was a few years ago.
''The health of our armed forces is good,'' the general says. Their condition , he adds, is ''the best that I have seen in almost 45 years in uniform.''
The problem, as he explains it, is that the Soviets are not slowing down. As a result, the goal of deterring war remains a difficult one.
''We are not going to have a very comfortable margin of safety at any time in the foreseeable future,'' he concludes. Even if the US spends every dime that Mr. Reagan requests for defense, this would provide only a ''more comfortable margin'' of safety, but not a ''considerable margin'' by the end of this decade.
Challenges from the Soviets are coming on almost every front. Even though the Soviet economy is only 55 percent as productive as the US, Moscow makes up for that by devoting a larger chunk of the budget to military output. US intelligence sources estimate that between 14 and 17 percent of Soviet gross domestic product goes to military purposes. In contrast, the US devotes about 6. 5 percent of its economic output to military use. Reagan would eventually like to boost that to about 7.5 percent.
The Soviet military has another advantage. While the US spends 43 percent of its military budget for personnel costs (salaries, housing, health care, retirement), the Soviets spend only about 11 percent, says Defense Secretary Caspar W. Weinberger. This leaves the Soviets a lot more money to buy tanks, planes, ships, and ammunition.
This is one reason that even with a record Reagan defense budget for 1985, the Soviets are expected to outproduce the US in military hardware. The US would build 720 tanks, the Soviets 2,300. The US would build 350 tactical combat aircraft, the Soviets 840. The US would build five major warships, the Soviets nine. The Soviets would also lead in armored vehicles (4,550 to 1,546), artillery (2,600 to 167), and submarines (10 to 5).
That kind of numerical advantage means the US and its allies need a technological edge to make up the difference. And that is one potential problem that the Pentagon sees ahead.
The latest military posture statement by the Joint Chiefs of Staff pinpoints a number of areas where massive Soviet investment in military technology and hardware is beginning to pay off.
The Soviets already have the edge in chemical warfare, antisatellite weapons, surface-to-air missiles, and ballistic missile defense. (They are improving their ballistic missile defense system around Moscow; the US shut down its only ballistic missile defense system years ago.) Further, the Joint Chiefs say the Soviets are closing the gap in a number of areas, including submarine-launched ballistic missiles, amphibious warfare, antisubmarine warfare, fighter aircraft, large ships, precision guided munitions, nuclear subs, and communications systems.
The Soviets are also seen gaining in a number of areas of important technology: electro-optical sensors, guidance and navigation systems, microelectronics and integrated-circuit manufacturing, radar, lightweight structural materials, and submarine detection. The Soviets have already caught up with the US in strategic missiles, antitank guided munitions, artillery, attack helicopters, infantry combat vehicles, naval mine warfare, and tanks. Their technology is about equal in aerodynamics, fluid dynamics, explosives, lasers, nuclear warheads, ocean science, and mobile power sources.
Every branch of the US military is feeling the pressure.
The challenge at sea. The US Navy's job will be getting tougher and tougher if current trends continue. The threat comes from a number of areas, including greater Soviet airpower, better Soviet submarines, and new and bigger Soviet aircraft carriers.
An American admiral who was recently asked which Soviet system he worried about most, had a quick reply: the Backfire bomber. With a 3,000-mile combat radius (even more with air-to-air refueling), the Backfire can swoop over a large portion of the world's oceans to threaten US fleets with long-range antiship missiles. Some 200 Backfires are currently in service, with 30 more being added every year. A new, larger bomber, the Blackjack, will be added by the Soviets in 1987.
One of the Navy's key jobs is to knock out the Soviets' 360 submarines as quickly as possible in time of war. But newer Soviet subs (five different types are currently under construction) are much quieter, and therefore harder to detect.
Adding to these challenges is the Soviet Navy's new emphasis on aircraft carriers. This year, the fourth Kiev-class Soviet carrier joins their fleet with its vertical-takeoff aircraft. But within a few years, the Soviets are expected to float their first big-deck, American-style carrier - which for the first time will give the Soviets the capability of launching offensive air operations far from their shores.
The challenge on land. Added to the well-known Soviet advantages in Europe (more tanks, more planes) is the threat of chemical warfare.
The Soviets devote 85,000 men to preparation for chemical warfare (the US, 7, 000). Their ships, vehicles, and key facilities are equipped with chemical warfare protection, while few American ones are. They are capable of delivering chemical weapons with tactical rockets, missiles, multiple-rocket launchers, cluster bombs, and other devices. The US has far less capability.
Why are the Soviets pushing these programs? US officials aren't sure, but they are gradually stepping up their chemical warfare training.
The challenge in the air. Control of the air over Europe will depend on pilot skill, advanced technology aircraft and munitions, and adequate numbers.
At present, the Western forces have everything in their advantage but numbers. There the Soviets lead. But the Pentagon has a worried eye on the new MIG-29 and Su-27, new supersonic, all-weather, night-capable fighters. These aircraft, says Mr. Weinberger, will ''significantly reduce our current tactical air advantage.''
In each of these cases, the Pentagon is aware of the challenge and is looking for ways to counter it. But the Soviets, as General Vessey says, are keeping up the pressure.