Ronald Reagan this week is scheduled to announce the most crucial military decisions of his presidency to date.

The stakes could hardly be higher, not only for Americans but also for people all over the world.

He must choose the strategic weapons most likely to keep fingers in Washington and Moscow off the nuclear button.

Ronald Reagan's challenge is deeper, broader and more complicated than the one faced by John F. Kennedy after he came to office 20 years ago decrying a "missile gap." Reagan, who believes the U.S. confronts a "window of vulnerability," is seeking to upgrade the whole nuclear "triad" of land missiles, bombers and submarine missiles, not just the land missiles Kennedy talked about in his 1960 campaign.

And, as Reagan explains his decisions, he will be playing to an extremely tough audience: voters and politicians in this country about to rebel over his spending more on guns and less on butter; protesters abroad who are saying the equivalent of "Yankee Go Home" to the new nuclear missiles Reagan wants to base in Europe; idealogues, military people, defense executives and fearful citizens who are looking for signs that their old champion is going soft on defense.

Reagan must decide what to do about the MX missile; B1 and Stealth bombers, and the missile submarine force. The specifics:

MX land missile. Where should this successor to the Minuteman ICBM be based? In the valleys of Nevada and Utah as the Air Force recommended? In the air aboard a giant plane? At sea in ships and/or submarines? Or, why not scrap it and build something smaller and cheaper and stick it in existing silos protected by anti-ballistic missiles?

All those options and more have been discussed. Many Air Force leaders are convinced they have won the argument, predicting Reagan will recommend for openers rotating 100 MX missiles among 1,000 cement garages scattered around government land near Las Vegas.

Chairman John G. Tower (R-Tex.) of the Senate Armed Services Committee said over the weekend that this "100-1,000" option looks to him like the front-runner. Defense Secretary Caspar W. Weinberger, even in closed congressional hearings, is not talking.

Pentagon research chief Richard DeLauer told reporters on Aug. 25 that Weinberger "wanted to delay making a decision on the final basing" of MX until more work was done on the new generation of anti-ballistic missiles for protecting land missiles, an option which would save Reagan money at this critical moment in his drive to reduce government spending.

Reagan has decried "the window of vulnerability" which U.S. defense officials say the Soviets opened by perfecting warheads accurate enough to blow up any U.S. land missiles standing still underground. So, the president will have to explain how his MX decision is closing that window.

But there will be a backlash because, as one Air Force general put it: "The MX is an ugly contest. No option is without problems. The question is which option is the least ugly."

The big danger of the Air Force's MX "shell game" is that it might provoke the Soviets into doubling their current arsenal of about 7,000 warheads by the end of the decade to cover all the missile's hiding places.

In response, the United States might feel compelled to expand MX and deploy a thick anti-ballistic missile system, with neither side feeling any more secure after spending billions of dollars. The better course, according to some specialists, would be to resume negotiations with the Soviets to limit the number of weapons each side has pointing at each other.

But Reagan appears to have bought the "bargaining chip" argument which calls for starting new weapons to show a stronger hand at the negotiating table.

Tower demonstrated on Sunday that this rationale is a driving force behind MX, declaring over Cable News Network that going ahead with the land version "will be a solid indication that we intend to build the system if we can't achieve arms reduction."

It has seldom worked out that way in the past, however.

U.S. officials in the early 1960s deployed the MIRV (multiple, independently targetable reentry vehicle) technology under the "bargaining chip" argument. MIRV is the technique of putting several nuclear bombs in the nose of one missile and sending each warhead to a different target.

The Soviets have since developed MIRV warheads which threaten today's land missiles, thus helping make the case for building the MX.

New bomber. Presidential candidate Reagan stood on the 1980 Republican platform which assailed former President Carter for canceling the B1 bomber in 1977. All signs point to Reagan reversing that decision by ordering an updated version into production. He is expected to promise to switch to the more advanced, radar-evading Stealth bomber when it is perfected.

The first B1 commitment is likely to be for 50 bombers, costing up to $200 million each, with another 50 promised if Stealth is not ready to take over the role of penetrating Soviet defenses and delivering the bomb in an all-out war.

But there are three big pitfalls in the brush here. One is that the B1 may be easy for Soviet defenses to down once it is flying in 1986 or so, just as former defense secretary Harold Brown has predicted in warning that up to $50 billion would be wasted on the plane.

The second is that the more advanced Stealth will be starved for funds to pay for the B1. Although almost everything about Stealth is labeled top secret, reliable sources said its development already is costing more than $1 billion a year, raising the question whether any administration can afford to pay for the B1 and Stealth at the same time.

Third, any super-weapon develops a political power base in Congress which fights to keep it in production. Once started, the B1 will be hard to stop no matter what the president says at takeoff.

Chairman Joseph P. Addabbo (D-N.Y.) of the House Appropriations defense subcommittee said he will try to delete any money Reagan requests for an updated B1 on the ground it would be obsolete by the time it flies. He doubts he could win this fight, adding: "I hope the order is for 50 B1s. That way there would be one for a museum in every state."

Backers of bringing back the B1 contend the aged B52s cannot safely be kept flying much longer and that, short of nuclear war, a new bomber would be needed to help win conventional battles in the oil-rich Persian Gulf.

Missile submarines. This is the easiest of Reagan's strategic decisions because of the widespread support for basing nuclear missiles under the sea. He intends to continue with the Trident submarine and its advanced Trident II missile, the D5.

The Trident II missile is powerful and accurate enough to destroy Soviet silos. It would be the heart of the so-called "common" missile some officials favor as an alternative to the MX. It would go into underground silos being vacated by aged Titan land missiles.

Reagan's imminent decisions on this strategic weaponry will mark the first phase of a "great debate" on the best way to keep the two scorpions in the bottle, the United States and the Soviet Union, from striking each other in the precarious years ahead.