Early in the Yom Kippur War of 1973, 50 Israeli tanks charged Egyptian infantrymen dug in near the Suez Canal. The Egyptians fired Sagger antitank missiles. Forty of the 50 tanks were knocked out.

That engagement, and others like it, inspired many military commentators around the world to declare the tank as dead as the cavalry horse. "Smart" missiles, they said, have returned the military advantage to the defense.

But U.S. Army leaders are now making a life and death bet that these commentators are wrong.

They are spending more money than ever before on a new tank; experimenting with a new typre of armored division built around it, and taking leaves out of the Israeli book for making the tank the deadliest weapon on the battlefield.

Army leaders conceded in a series of interview that the Yom Kippur War demonstrated that an infantry armed with a smart weapon could knock out a tank almost as easily as a rifleman could shoot the horse out from under the cavalryman.

But, these leaders insisted, the tank need not and should not go the way of the horse. The Yom Kippur War, they said, actually demonstrated that the tank is still the lion of the battlefield if used properly.

During the first day of the Yom Kippur War, said Gen. Donn A. Starry, the U.S. Army's "Mr. Tank," the Israelis made the mistake of charging against Egyptian infantry without supporting fire from their infantry, artillery or planes.

This meant that the Egyptian soldier, instead of having to keep his head down, could take careful aim at the advancing tank and then guide his Sagger into it by control signals he sent along the wires trailing from the Soviet-made missile.

"One of the great lessons of that war," said Starry, commander of the Army Training and Doctrine Command at Ft. Monroe, Va., "is that there is not single system that's going to dominate the modern battlefield." Winning requires a combination of well-trained armor, infantry, artillery and aircraft.

"If you get away from the concept of a combined arms team," Starry continued in the interview, "you're liable to get yourself in trouble."

As for the killing power of smart antitank missiles, Starry said they accounted for only about 15 per cent of Israel's tank losses in the Yom Kippur War. Unlike tanks, infantrymen armed with the smartest missiles, he said, cannot live through the hail of artillery shrapnel.

Israel's post-audits of the Yom Kippur War seem to agree with the U.S. Army's. Said retired Israeli Lt. Gen. David Elazar, for example: "The tank was the backbone of the land forces during this war and will remain so in the future."

Their faith in the tank thus buttressed, Army leaders are currently eager to get their new XM-1 main battle tank into the field. Chrysler has finished two prototypes and will start deliveries of production models in 1980.

The Army plans to spend $4.8 billion to by 3,323 XM-1 tank. Army officials said improved version of their best tank now in service, the M-60, cost about $500,000 a copy compared to $1.5 million for each XM-1.

But will the XM-1 - faster, better armored, more sophisticated all the way from its turbine engine to devices that enable to fight at night - have triple the combat effectiveness of the M-60 to justify its triple cost?

When queried, an Army spokesman said comparisons of combat of effect said comparisons of combat effectiveness are secret. An experienced and high-ranking Army officer said "this is the real issue" about the XM-1, "not whether it bakes cookies on cold days."

The officer said that on the basis of his experience as a commander in Europe, the M-60 tank delivers only 60 per cent of its full combat potential because crews keep changing: there is not enough training space or time in West Germany, and maintenance problems persist.

Because the XM-1 will be an even more complicated tank than the M-60 for the crew to operate, he continued, the new tank could deliver even less than 60 per cent of its combat potential unless sweeping improvements are made in crew selection, stability and training.

Brig. Gen. Frederick C. Brown, who comands the training school at Ft. Knox, Ky., where Army officers learn how to right with tanks, said in an interview that he and fellow training leaders are aware that it is the people more than the machine, who decide who wins or loses on the battlefield.

In Vietnam, Brown said, it was "a 50 per cent battlefield" for tankers, meaning that if they performed sloppily there still was only a 50-50 chance of getting blown up. He called Europe, in contrast, "an 80 to 90 per cent battlefield," meaning if the tanker messes up, the almost certainly will get killed.

At Ft. Knox, the Army is lengthening its training course for M-60 tankers and organizing a new course for the XM-1.

At Ft. Hood, Tex., the Army is conducting mock battles to test ideas for giving and armored division more punch and survivability, including increasing the tank crew from four to five in case one man gets hit; folling the Israeli lead by reducing a tank platoon from five tanks to three to make the force easier to manage; combatting the massive firepower from Soviet artillery; establishing antitank missile companies within tank battalions.

Putting fewer tanks in each platoon, critics agree, would make each one easier to manage. But to give the brigade commander his accustomed number to tanks would mean overwhelming him with more individual units than he could direct.

So far, Defense Secretary Harold Brown is letting the Army pursue the dream dating back to 15th century inventor Leonardo da Vinci. Da Vinci wrote that "I am building secure and covered chariots, which are invulnerable.And when they advance with their guns into the midst of the foe even the largest enemy masses must retreat, and behind them the infantry can follow in safety and without opposition." CAPTION: Picture, One of two prototypes of the XM-1 main battle tank Chrysler has built for the Army with a price tag of $1.5 million each for the 3,323 the Pentagon plans to buy.; Illustration, Leonardo da Vinci's design for a "secure and covered chariot." The Washington Post