STATE DEPARTMENT officials have been warning for nearly two years that Syria’s civil war, if not brought to a prompt end, could blossom into a regional conflagration that consumes Iraq and Lebanon and threatens vital U.S. interests. Their predictions have been coming true, but in slow motion, enabling those who hope to ignore the growing danger — notably President Obama — to remain complacent.

This week brought another potential wake-up call, in the form of a disturbing escalation of terrorist violence in both countries. In Iraq, al-Qaeda launched an offensive to take control of two cities, Fallujah and Ramadi, that U.S. troops sacrificed heavily to clear of terrorists between 2004 and 2008. In Lebanon, a car bomb exploded in a Hezbollah-controlled suburb of Beirut just days after a prominent critic of the Shiite movement was assassinated in another bombing.

A multi-sided and transnational war is developing among the same factions battling in Syria: al-Qaeda; Shiite Hezbollah and its sectarian allies in the Syrian and Iraqi governments; and more moderate Sunnis who are resisting both al-Qaeda and the regimes. Across the region, al-Qaeda is surging. At the time of the U.S. withdrawal from Iraq, the Sunni jihadist movement had been all but extinguished, thanks to the collaboration of U.S. and Iraqi forces. Now it controls a wide swath of territory in eastern Syria that adjoins Iraq’s Anbar province, where Fallujah and Ramadi are located. In Lebanon, too, an al-Qaeda-linked force called the Abdullah Azzam Brigades is believed responsible for multiple attacks, including twin bombings outside the Iranian embassy in November.

Hezbollah played a decisive role in spreading the sectarian war by sending thousands of its fighters to Syria to defend the regime of Bashar al-Assad. The Shiite-led government of Iraq, too, has fanned the flames through its attacks on protest encampments set up by more moderate Sunni opponents in Fallujah and Ramadi. Meanwhile, foreign Sunni fighters have poured into Syria and western Iraq. According to a study by an Israeli think tank released Thursday, the vast majority of the 6,000 to 7,000 militants to infiltrate the area have joined al-Qaeda-linked groups, including more than 1,000 from Western countries.

For Mr. Obama, the presence of al-Qaeda has been a reason to withhold U.S. aid to rebels fighting the Assad regime and to reject more forceful measures to bring the war to an end. That policy has left his administration without a strategy for preventing the terrorists from consolidating a safe haven in Syria and extending their influence to Lebanon and Iraq — where the gains painfully won by U.S. troops are being reversed. The administration has supplied some arms and intelligence to Iraqi government forces fighting al-Qaeda, but that is little more than a palliative. Sooner or later the United States will have to face the threat to its vital interests emerging across the Levant.