Iraq's bold invasion of the rich and vulnerable Kuwait was also a strike at the interests of the United States and many of its allies around the globe who have long sought stability and a balance of power in the Persian Gulf to keep the world's oil lifeline pumping.

The invasion will put new pressure on American ally Saudi Arabia, intensify fears of a wider regional conflict, and drive up world oil prices at a time when the U.S. economy is already teetering on the brink of recession.

Although only a few days ago Bush administration officials said the U.S. had no interests in an Iraqi-Kuwait dispute over borders and oil pricing, the invasion has thrown into high relief new questions about how far the United States is willing to go to defend its Arab allies. For the first time since President Carter declared U.S. stategic interests in the Gulf, U.S. officials may now be forced to define precisely the threshold for American action, and do so at a time when the Soviet threat to the oil fields, which was long used as a justification for a U.S. presence, seems to have dissipated.

Without attractive military options at the outset of the crisis, the Bush administration yesterday rejected Kuwaiti pleas for intervention by force, and instead responded to the invasion by seeking to isolate Iraq internationally. President Bush condemned "this naked aggression" and froze assets of the protagonists; the U.S. pressed other nations to halt Iraq's supply of weapons and staunch its trade. A State Department official said that in economic and monetary terms, "We want to bring the world down on their heads."

Although U.S. intelligence reports had carefully tracked the Iraqi forces amassing on Kuwait's border and said that an attack was possible, senior officials said they were nonetheless taken aback by the swiftness of the invasion. "Nobody was sure he was going to pull the trigger," a high-ranking administration official said of Iraq President Saddam Hussein. Assistant Secretary of State John H. Kelly had met with Iraq's ambassador to the United States at the State Department Wednesday afternoon to urge caution, but the ambassador said nothing, according to U.S. officials. When the troops raced across the border, Kelly summoned the ambassador back to the department at 11:30 p.m. for a strenuous protest, officials said.

Kuwait's ambassador to the United States, Saud Nasir Sabah, said yesterday that "we knew" about Iraq's concentration of troops on the border, "but we have been assured by prominent heads of state all along that the Iraqis have given their word that they will not use military force."

The suddenness of the invasion scrambled plans of President Bush, who shortened a planned visit to Colorado, and Secretary of State James A. Baker III, who cut short a visit to Mongolia and -- in an unusual twist -- headed for Moscow to join Soviet Foreign Minister Eduard Shevardnadze in a joint denunciation of the attack.

According to many specialists in Gulf affairs, Saddam has more than ever become an ominous threat to the goal of stability that has been a U.S. priority in insuring the oil supply. This was part of the rationale behind the 1987 decision to reflag 11 Kuwaiti tankers and protect them with American forces in the midst of the Iran-Iraq War. Kuwait was then a major contributor to Iraq's war effort and its tankers were being targeted by Iran; the U.S. was implicitly helping Iraq in order to prevent Iran from winning a chokehold on the world's oil. Kuwait originally had asked the Soviets for help protecting its shipping, and the U.S. response was aimed at blocking any Soviet presence in the Gulf.

Now, with Iran in disarray, it is Iraq that threatens the U.S. goal of stability. "The key issue is precisely the same as it was in the 1970s and 80s -- the United States does not wish to see a major adversary control the assets of the Gulf, whether it is the Russians or the Iranians or now the Iraqis," said Geoffrey Kemp of the Carnegie Endowment for International Peace.

"I think we are very badly shaken in our strategic design for the region," said Michael Hudson of the Georgetown University Center for Contemporary Arab Studies. "We have predicated our strategy for having access to inexpensive oil and security in this region on a special relationship with certain key countries." Iran once played this role, he recalled, and Saudi Arabia still does.

"The perplexity sets in now as we begin to suspect that Saudi Arabia has been knocked down a peg," he added, both at last week's Organization of Petroleum Exporting Countries meeting, where Iraq's demands on oil quotas prevailed, and in the invasion of Kuwait on Saudi Arabia's border.

Hudson said many had underestimated Iraq's potential for a rebound after the war with Iran. "I think some of us expected, however the war ended, even if the Iraqis won, they would never be able to do very much major damage elsewhere, to the south and west -- the Iranians would be there," he said. "We expected they would behave themselves. What we didn't calculate was the extent Iran was bled and debilitated by the war."

If Iraq retains control of Kuwait, it will have effectively doubled its proven oil reserves, putting it second in the Gulf behind Saudi Arabia. Tom McNaugher, a senior fellow at the Brookings Institution, said this could lead Saddam to attempt to "intimidate" the Saudis. "When you have demonstrated your willingness to invade a country that hasn't been invaded in 200 years, you make a point that you play a brand of hardball that's new to the region," he said.

While Kuwait had no defense treaty with the United States and sought to play on the superpower rivalry during the Cold War, relations with Washington had improved in recent years. Kuwait held firm against demands that it release Shiite Moslem terrorists involved in embassy bombings; it had also stood up to Iran.

"One could understand why the Kuwaitis will feel bitter if we don't do more than we have done," said Hudson. "It looks like nice guys finish last."

Staff writer George Lardner Jr. and staff researcher Bruce Brown contributed to this report.