BAHRAIN HAS BECOME the hidden story of the Arab Spring. While the popular uprisings in Libya, Syria and Yemen have dominated the news in recent months, far less attention has been paid to the tiny but strategic Persian Gulf emirate, which hosts the U.S. 5th Fleet. That’s partly because Bahrain’s ruling al-Khalifa family deflected criticism from the massive crackdown it launched in March by promising to initiate a dialogue with its opposition and implement political reforms. The regime, however, hasn’t delivered — and now it is risking a new explosion of unrest that could destabilize not just Bahrain but the region around it.

The latest trouble began with the promised National Dialogue, which unraveled soon after it began in July. The government gave the largest opposition party five out of the assembly’s 300 seats and left some crucial reform issues — such as the reform of parliamentary districts — off the agenda. Most of the opposition walked out before the “dialogue” concluded with several minor recommendations. One of them would increase the powers of the regime’s principal hard-liner, Prime Minister Khalifa bin Salman al-Khalifa, who has been in office since 1971.

Another conciliatory initiative, a commission to investigate the unrest, has been undermined by the behavior of its Egyptian chairman, who has made public statements preemptively exonerating the ruling family. A promise to rehire thousands of workers fired from their state jobs because of their suspected support for the opposition has been only partly fulfilled. And while some political prisoners have been released — a group of doctors were freed this week after they and other prisoners staged a hunger strike — hundreds remain jailed and the regime continues to use a “court of national safety” to imprison opposition leaders.

Rather than moving toward reconciliation, Bahrain is more polarized than ever, and the fault line increasingly falls between the ruling Sunni elite and majority Shiite population. Clashes between protesters and police occur almost every night in Shiite villages, and the Aug. 31 death of a 14-year-old boy who the opposition says was struck by a tear gas canister has magnified the tension. Thoughtful Bahrainis worry that a new eruption of mass protests is imminent and that it may lead to a purely sectarian conflict that could spread to Saudi Arabia and even Iraq.

The United States has considerable leverage in Bahrain — through the 5th Fleet, military aid programs and a free-trade agreement. But the Obama administration has been timid here as elsewhere during the Arab Spring. In May, President Obama made a strong statement about Bahrain during a speech on the Middle East in which he promised to support the cause of democratic change across the region. But there has been no follow-up; no senior U.S. officials have visited Bahrain in months, and the administration has had nothing to say about the deteriorating situation. This is shortsighted: If Bahrain blows up, vital U.S. interests will be at risk. The administration should use its influence now — before the crisis resumes.