WASHINGTON — There was a time when Saudi and American interests in the Middle East seemed so aligned that the cigar-smoking former Saudi ambassador, Prince Bandar bin Sultan, was viewed as one of the most influential diplomats in Washington.

Those days are over. The Saudi king and his envoys — like the Israelis — have spent weeks lobbying fruitlessly against the interim nuclear accord with Iran that was reached in Geneva on Sunday. In the end, there was little they could do: The Obama administration saw the nuclear talks in a fundamentally different light from the Saudis, who fear that any letup in the sanctions will come at the cost of a wider and more dangerous Iranian role in the Middle East.

Although the Saudis remain close American allies, the nuclear accord is the culmination of a slow mutual disenchantment that began at the end of the Cold War.

For decades, Washington depended on Saudi Arabia — a country of 30 million people but the Middle East’s largest reserves of oil — to shore up stability in a region dominated by autocrats and hostile to another ally, Israel. The Saudis used their role as the dominant power in OPEC to help rein in Iraq and Iran, and they supported bases for the American military, anchoring American influence in the Middle East and beyond.