MANY EXPERTS believe the ongoing upheaval in the Middle East will be the most important foreign policy test of Barack Obama’s presidency. Yet this president did not come into office at an easy time — and the critical challenges that predated the Arab revolutions have not disappeared. Strong reminders of that came this week in two news stories reported by The Post: Pakistan’s demand that drone attacks against al-Qaeda and Taliban militants in its frontier territories be scaled back; and Iran’s claim of fresh progress in its nuclear program.

The reports from Iran are particularly disturbing because its Islamic regime has been a short-term beneficiary of the revolution in Egypt and unrest in the Persian Gulf emirate of Bahrain. Deposed Egyptian strongman Hosni Mubarak was a determined enemy of Iran, and Bahrain’s crackdown on popular protests threatens to touch off the Shiite uprising, there and in eastern Saudi Arabia, that Iran has long wished for. As important, the region’s turmoil has pushed up oil prices, making it easier for Iran to endure the economic sanctions painstakingly orchestrated by the Obama administration.

The revolution in Egypt began days after the regime of Ali Khamenei bluntly rejected the latest attempt by a six-nation coalition to begin negotiations on the nuclear program. Since then there has been no sign of diplomatic activity, but Iran has been busy expanding its nuclear capacity. In recent days officials announced that tests of a new generation of centrifuges for enriching uranium had been successful, and that a Russian-built nuclear reactor would begin operations early next month.

The progress on centrifuges is significant because Iran until now has relied on slow and inefficient centrifuges, many of which appear to have been damaged by software sabotage. The more advanced machines, The Post’s Joby Warrick reported, could work at least six times faster. Iran has already enriched more than 3,600 kilograms of uranium to a low level, enough for two nuclear bombs with further processing. The faster centrifuges mean that were Iran to embark on a “break-out” strategy — a race to complete a bomb — it could do so far more quickly, if it manages to install a significant number of the new machines.

Several months ago, administration officials were speaking confidently of an Iran that, pinched by sanctions and hamstrung by problems in its nuclear work, seemed ready to begin talks. Now the talks are off, the economic pressure is easing and the nuclear work once again could be gaining momentum. Yet the administration seems to have no clear alternative to its long-standing strategy of waiting for the regime to negotiate.

The better course, which we among others have urged since the opposition Green Movement was born nearly two years ago, is to bet on a renewed popular uprising in Iran. President Obama recently made a gesture in that direction with a video address to Iranians that denounced government repression and said young Iranians had the “power to forge a country that is responsive to your aspirations.” But there is much more the administration could do, such as finding ways to support Iranian unions and student movements, stepping up broadcasting and accelerating funding for technology that can undermine Internet censorship. Passivity is a dangerous option; while the world watches the Middle East, Iran’s drive for a bomb relentlessly continues.