When Saudi Arabia's rulers invited the United States military into the kingdom yesterday to help defend it against Iraq, they violated a long Saudi tradition of avoiding direct confrontations with powerful Persian Gulf neighbors and of keeping would-be Western protectors at arm's length.

Many Americans, who for more than a decade have watched bruising political battles in Congress over sales of AWACS radar planes and other military aid to the oil-rich kingdom, may be inclined to think of Saudi Arabia as a powerful nation -- and to wonder why it took the Saudis so long to take tough moves to defend themselves. But, according to scholars of Saudi affairs, the country's rulers do not see themselves that way.

In the roughly 65 years since the Saud family united the rivalrous tribes of the Arabian peninsula and named the country after themselves, they have been intensely cautious in the welter of conflicts that have swirled around their borders. In part, according to scholars, this is because of Saudi perceptions of their own weakness, and in part, because of cultural traditions.

"The Saudis have the appearance of dominating the Arab world because they have a lot of money," said Michael Hudson, a professor at Georgetown University's Center for Contemporary Arab Studies. But "they don't have a lot of power."

The Saudis, in a formulation heard often in the kingdom, wanted the United States and its military might "just over the horizon" -- close enough to protect them in a crisis, but not so close as to become embarrassing.

Yesterday's move by the Saudis was significant, analysts said, because it broke not simply with their policy of staying aloof from their superpower patron, the United States, but with the cultural traditions of reticence and indirectness that nurtured their policy.

While inhospitable coasts and Arabian desert that make up the peninsula have for centuries afforded protection from invasion by outsiders, they also help keep Saudi Arabia one of the world's most thinly populated countries. With an area comparable to the United States east of the Mississippi, Saudi Arabia has only 8 million people.

During this century, however, advances in military technology have undermined the kingdom's natural defenses, leaving the country to worry over its demographic weaknesses. Neighbors such as Iraq, Iran, Syria and Egypt have been able to build powerful military machines on the basis of manpower, each putting between 400,000 and 1 million troops under arms, according to The International Institute for Strategic Studies in London. Saudi Arabia, struggling to build forces of little more than 70,000 men, in recent decades has tried to buy security by purchasing sophisticated weapons and maintaining close but quiet links with Washington.

Even the more modest Saudi forces have had to depend on foreign manpower -- a particular discomfort for Riyadh, given the Saudi Bedouins' cultural wariness of outsiders. Reluctant to take in such foreigners at all, Saudi Arabia years ago turned to Moslem Pakistan, under its former military leader, Mohammed Zia ul-Haq, to supply ground forces to supplement its own troops.

In 1983, Saudi Arabia agreed to a joint defense pact with Kuwait and four other gulf sheikdoms under the Gulf Cooperation Council, partly in an effort to avoid the appeal to Western forces that was announced yesterday.

To make up for their natural vulnerability, the Saudis have become expert at a cautious style of diplomacy that avoids confrontation with its neighbors, even in a crisis.

"To have to go one way or the other {in a conflict} has always been difficult for them," said Hudson. "Given their essential thinness and their vulnerability, they have always liked to prevent any alliances . . . between their bigger neighbors."

The current Saudi dynasty has pursued such a balance-of-power strategy since its birth amid the pre-World War I struggle between Britain and Ottoman Turkey to dominate the Middle East.

Similarly, in the recent U.S.-Saudi relationship, Riyadh has sought Washington's aid when needed, but -- until yesterday -- has stopped short of inviting any U.S. offensive capacity on its soil. During the eight-year Iran-Iraq war, Saudi Arabia consistently tried to avoid direct conflict with a hostile Iran, breaking ties only after Iran backed an uprising in Mecca in 1987 and attacked its ships. During the war, the Saudis allowed the United States to fly AWACS surveillance planes from Riyadh, but insisted on running the operation jointly, partly in the form of training for the Saudi Air Force.

In 1987, when the war spilled into the Persian Gulf, threatening Kuwaiti exports (and, ultimately, oil production in Saudi and other offshore fields) the United States moved Marines, and naval and air forces in to protect its ally's oil trade.

Despite the importance of the U.S. operation for Saudi interests, the kingdom refused to give Washington permission to base its Navy or Marines on Saudi territory. The Navy was forced to conduct many of its operations from seagoing barges anchored in the gulf, an arrangement some military observers saw as vulnerable to Iranian attack.

In Riyadh's shift yesterday, according to Hudson, "one factor that may have weighed in the decision, which was not taken easily, is the collapse of the Kuwaiti monarchy."

The fall of Kuwait's traditional tribal sheikdom, similar in its history and claims to authority to the House of Saud, "poses serious longer term questions about the legitimacy and viability of the others, including their own," Hudson said. "This is more serious for them than anything they had to confront directly during the Iran-Iraq war."