In reaching back half a century to the battle against Hitler and fascism, President Bush yesterday sought heroic and reassuring precedent for an action that many of his predecessors have taken with unhappy results -- the making of a new American commitment to protect a piece of real estate far from home.

By sending an expeditionary force that may grow to 50,000 men to Saudi Arabia, Bush dramatically formalized an American security interest in the oil fields of the Middle East. Many past presidents have alluded to that interest and it was enshrined in the "Carter Doctrine." But whereas earlier presidents, including Jimmy Carter, merely said they would fight to protect the flow of oil from the Persian Gulf, Bush is now sending troops prepared to do so.

The mission is risky. Though Bush is sending a large force, it is a small fraction of the size of the Iraqi army, and is unlikely by itself to achieve the goals Bush enunciated -- particularly the withdrawal of Iraq from Kuwait.

Bush declared that the independence of Saudi Arabia and its neighbors in the gulf is a "vital interest" of the United States. But that independence is threatened by an unpredictable and brutal leader who has now invaded two of his neighbors -- first Iran and now Kuwait -- without provocation. Saddam Hussein has shown himself willing to use the most reviled methods to achieve his ends, from indiscriminate attacks on civilian targets and commercial shipping to poison gas.

In his address to the nation yesterday, Bush never once referred to Saddam as Hitler. Nor did he explicitly cast himself as Franklin Roosevelt or Winston Churchill. But his references were clear.

Iraq's assault against Kuwait was nothing less than a "blitzkrieg." America was fighting not merely for the control of its oil supplies; it was battling for freedom against aggression.

The analogy to a time long ago, when few doubted that the United States served the cause of righteousness, was irresistible. It was also a bold effort to push aside other memories.

For in sending troops to Saudi Arabia, the president was committing the country to a region that has been the source of much pain and trouble in recent American history. If the United States was successful in battling for freedom in Europe, it retreated from Iran and Lebanon with ashes in its mouth. In embracing the memory of Roosevelt, Bush was trying to exorcise the ghosts of more recent presidents, of the Iran-hostage crisis and the 241 dead Marines in Lebanon.

Bush was trying to exorcise another ghost as well. In making what could be a long-lasting commitment to the defense of Saudi Arabia, Bush was directly challenging an American aversion to protracted engagements that dates to Vietnam.

Ever since Vietnam, Americans have been wary of analogies to Munich, the infamous sell-out of Czechoslovakia to Hitler, because memories of that appeasement were invoked so insistently in support of the country's commitment to Indochina.

It is a sign of how much the world has changed that Bush could invoke the Munich analogy without apology. "Appeasement does not work," Bush declared in his address to the nation yesterday. "As was the case in the 1930s, we see in Saddam Hussein an aggressive dictator threatening his neighbors."

The lesson, said Philip Khoury, a Middle East specialist at the Massachusetts Institute of Technology, is clear. "Vietnam," he said, "is over."

"We don't really have to worry anymore about what people will say about deploying U.S. troops," Khoury went on. "We've all become a little more conservative."

But if Americans have become more conservative, they are still impatient when it comes to foreign commitments.

Rep. Charles E. Schumer (D-N.Y.) noted that while the president now enjoys overwhelming support , "the American people would start feeling differently" if the commitment to Saudi Arabia became open-ended -- and especially if the United States became involved in "a long-term land war." In other words, the Vietnam legacy may persist after all.

But if Vietnam was a case where America's stake was never clear, the threat to Middle East oil fields is easy for everyone to understand.

"There's a lot more at stake here than $30 oil," said Walter LaFeber, a historian at Cornell University. "This is about the ability of one person to manipulate 45 percent of the oil reserves in the world." LaFeber, a longtime critic of American interventionism, said he found himself shocked to be supporting the president's policies.

Alan Brinkley, a historian at the City University of New York Graduate Center, argued that America's commitment to Saudi Arabia was decidedly different from earlier U.S. interventions in the Middle East. These included the 1958 Marine landing in Lebanon that successfully bolstered an anti-communist, Christian-led government and the far less successful Marine commitment that began in 1982. Not all of the differences are comforting.

"Previous interventions weren't particularly risky," Brinkley said. "In the past, we were going into situations where, relatively speaking, the opposition was pathetically weak." In facing Iraq, on the other hand, "we've gone in against a military that might be a match for American power."

There were other differences, too. "This is the first time we've gone into the Middle East for reasons having nothing to do with communism," he said. "And this is the first time a plausible case can be made that this is an intervention on behalf of vital economic interests."

There is, in fact, something thoroughly old-fashioned about America's interest in the Midle East oil fields. The United States' commitment to Saudi Arabia is akin to the straightforwardly commercial behavior of 19th century powers.

"This is bald self-interest we're talking about here," said Tom Mann, director of governmental affairs at the Brookings Institution. "And in some ways, Bush's way of dealing with these Middle Eastern countries is almost colonial in character."

Mann emphasized that he said this as a backer of Bush's moves, but even supporters of the American commitment fret that many Arabs will make the colonial analogy, too, and not much like it.

"I really do worry about the high level of resentment toward America in the Arab world," said Khoury. He noted that recent visitors to the Middle East among his academic friends "have never felt as uneasy about being Americans in the Arab world as they do now."

But whether Bush's moves prove popular in the Arab world or not, an American military commitment to preserving the conservative regimes of the Middle East has been in the cards since 1979, Khoury said, when the shah's regime in Iran collapsed. The Carter Doctrine was a direct response to this loss of regional influence.

And so President Bush has taken a logical, if dangerous, next step. In doing so, he appears to have popular support. But the mood is somber, anxious, realistic. There was little in the way of a patriotic outpouring and far less vocal hostility to Saddam than there was to Ayatollah Ruhollah Khomeini.

The president cast the United States' mission in bold terms. But Americans, well aware of their country's problematical history in the Middle East and wary of long-term commitments, may be satisfied with much less: For most of them, avoiding yet another disaster will probably be enough.