Two months into the Arab revolution, one very fat lady has yet to sing. But the turn of Saudi Arabia — home to one-fifth of the world’s oil reserves, and the United States’ most important remaining Arab ally — may be coming soon.

Think there’s no chance that this kingdom’s restless youth — 60 percent of the population is under 18, and 28 percent of working-age youths are unemployed — will rise in revolt? King Abdullah bin Abdul Aziz doesn’t agree with you. On Wednesday, the ruler landed in Riyadh after a three-month absence abroad for medical treatment — and for an 87-year-old with a bad back, he looked like a man in a big hurry.

Before his plane even touched down, Abdullah had ordered up $36 billion in new welfare grants for his people — about $2,000 for every Saudi. There were loans for young Saudis to buy homes, get married and start a business, and a 15 percent pay raise for government workers. Next are a prisoner release and a cabinet reshuffle.

Meanwhile, waiting among the 50 or so white-robed men on the tarmac to meet Abdullah was the man who worries him most: King Hamad bin Isa al Khalifa of the neighboring island nation of Bahrain. A week ago the Khalifa regime tried to put down the first popular uprising in an Arab emirate by force — the solution sought by Saudi Arabia. It failed, thanks in part to countervailing pressure from the United States, which keeps a fleet in Bahrain’s port.

Thousands of protesters are camped in the center of Bahrain’s capital, Manama. Their demands, from the Saudi perspective, are frightening: at the least, a constitutional monarchy that will empower the country’s repressed Shiite majority — and maybe also the deposal of the al Khalifa family, which is Sunni. Watching closely are the 2 million Shiites of Saudi Arabia’s oil-rich eastern province, who are also a disadvantaged majority in their region and who are separated from Bahrain by a 16-mile causeway.

King Hamad probably has broken some bad news to King Abdullah: I no longer have the option of ending this by force. It won’t work — and the Americans won’t let me. That leaves the Saudi ruler with a couple of hard choices. He can order Saudi forces through the causeway to put down the Bahraini Shiites, in what would be an Arab version of the Soviet invasion of Czechoslovakia. Or he can let the al-Khalifas bargain away their power, while hoping that the democratic infection doesn’t spread.

The invasion is a real possibility: Saudi troops helped put down a Shiite rebellion in Bahrain in the 1990s. Earlier this week, the Saudi Council of Ministers issued a Brezhnev-like declaration: “The kingdom will stand by the sisterly state of Bahrain with all its capabilities.” A lot of experts in Washington are convinced that the Saudis won’t hesitate to act if the Bahraini regime appears in jeopardy.

But invasion could bring Saudi Arabia directly into conflict with the Obama administration, which is backing the reform route in Bahrain. It could even cause a historic rift in the 65-year-old alliance. At the least, a $60 billion arms sales package just agreed to between Washington and Riyadh would be in danger.

Abdullah has no love for Obama; he spurned the U.S. president’s request for help in the Arab-Israeli peace process and fumed at Obama’s turn against Egyptian strongman Hosni Mubarak. According to the New York Times, the last of their two phone calls during the Egyptian crisis “ended in sharp disagreement.”

Still, I’m betting that Abdullah would rather be a Gorbachev than a Brezhnev. Rather than invade, he’s more likely to embrace the strategy of trying to get ahead of the Arab wave of change before it is too late.

That’s because Abdullah has started down this path before. After the attacks of Sept. 11, 2001, in which 15 Saudis participated, the then-crown prince began to cautiously plan a liberalization of his economic and political system. One of his closest counselors was a 40-ish Georgetown graduate named Adel al Jubeir — who, since 2007, has been the Saudi ambassador to Washington.

In a 2003 interview, Jubeir outlined to me an expansive political reform agenda: first, elections in professional organizations of journalists and doctors, as well as universities; then municipalities. Last would come an election to the quasi-parliamentary shura council, which Abdullah now appoints. “If we move deliberately and we do all the right steps, I don’t see why we can’t have a society with the rule of law and civil liberties and elections,” Jubeir said.

The municipal elections were duly held in 2005; in 2009, when another vote was due, they were canceled. Abdullah’s reforms, undertaken in large part because of pressure from the Bush administration, stalled. But Jubeir is still around — in fact, the king just extended his term in Washington. Is the fat lady finally ready to sing? We’ll soon find out.