Washington ? The United States and Saudi Arabia have arrived at a familiar moment in a love-hate relationship based on oil and cultural incompatibility. The two governments normally take a deep breath and decide that the benefits their strategic entanglement brings outweigh the problems it creates. But that stock-taking is not happening this time.

Problems are proliferating instead of being quietly disarmed by adroit diplomacy. Washington and Riyadh are slipping toward new confrontations over the future of Iraq, democracy in the Middle East and U.S. objections to Saudi financing and support for Arab terrorist groups.

These explosive issues already spin beyond the control of the diplomats and encourage the political leaders on each side to contemplate a silent but deadly thought of last resort: We can get along without you.

Containing the damage to this still useful relationship — while prodding Saudi Arabia to change its most egregiously intolerant and incendiary ways — tops the list of urgent and difficult challenges that Colin Powell’s State Department confronts. Yet the Saudi challenge arrives as American diplomacy seems destined to play a rapidly diminishing role in world affairs.

The current guessing game of how long Powell will remain as secretary of state is much less important than the question of whether he and his successors can adapt and be effective in the world shaped by 9-11 and the strategy of military pre-emption it provoked.

A Washington Post story reporting that Powell’s deputy and closest friend, Richard Armitage, recently told the White House that the two would not be around for a second Bush term smoked out strong official denials this week that hint at the true state of affairs: The only people Powell and Armitage have not told about their intentions seem to be those who work at the White House.

They have told co-workers in Washington, and relatives, bankers and foreign policy sages in New York. Powell had made indirect but clear public references to his plan to leave, and astute diplomats based here concluded a month or two ago that they were working with a lame-duck chief diplomat 18 months before the end of George W. Bush’s electoral mandate.

This burbling about his tenure handicaps Powell in achieving a meaningful legacy in whatever time he has left. That legacy should include the vital task of updating the U.S.-Saudi relationship to reflect the changes brought about by 9-11, the American military victory in Iraq and what one Bush aide calls a “generational commitment” by Americans to transform the Middle East into a more democratic and pacified region that does not directly threaten American lives or interests.

The agenda of meaningful U.S. diplomacy was not advanced by the spectacle of Saudi Arabia’s foreign minister, Saud Faisal, coming to Washington on July 29 to upbraid both Congress and the president for spreading “misguided speculation” about Saudi Arabia’s role in financing global terrorism. Such an appearance is the very definition of diplomatic failure, and a sign of an incipient crisis in relations between two capitals that no longer feel they can rely on each other.

U.S. commitments to protect the Saudi royal family and oil fields from revolution or other disaster stretch back to 1944. But the involvement of Saudi citizens in 9-11, the kingdom’s relative decline as the swing factor in world oil markets and the Bush agenda for regional change means that Washington is no longer willing to overlook Saudi behavior that was left alone in the past.

“The old assumption was that the cost of terrorism was sustainable,” says one senior administration official. “The horror of 9-11 showed that it is not. You have to deal with the threat now before it strikes you.”

Prince Saud did not mount his soapbox here believing he could force Bush to declassify 28 pages of classified material in a congressional report that fingers Saudi financing for charities that support terrorist groups. A public release that would provoke official arguments over the report was the last thing the Saudi diplomat wanted. He was here as a politician, to show Saudi opinion that he was standing up in the belly of the monster.

“I understand the dangers,” a senior Saudi official said to an American friend a few years ago. “But people in the kingdom think these contributions buy them paradise in the hereafter and protection here on Earth. It will not be easy to get them to change.”

Guiding other nations to make difficult but needed changes is the heart and soul of diplomacy. How well Powell and his diplomats can do this in Saudi Arabia is now a crucial test to his still-unformed legacy.

Jim Hoagland is a columnist for Washington Post Writers Group.