FOR MONTHS, SOME Obama administration officials have argued that it would be unwise to supply arms to Syrian rebels because doing so could fuel a sectarian war that could lead to the empowerment of extremists and spread beyond Syria’s borders. The United States has consequently withheld lethal aid — only to watch a deepening war in which al-Qaeda and other jihadist groups are gaining ground, while Shiite-Sunni clashes have steadily escalated in Iraq and Lebanon.

The latest twist in this downward spiral came Sunday in Iraq, where a wave of bombings and attacks by Sunni insurgents, aimed mostly at Shiite civilians, killed more than 100 people in 10 cities. At the same time, a Baghdad court convicted the country’s Sunni vice president of murder and sentenced him in absentia to death, in what most Iraqi Sunnis regarded as an act of sectarian aggression by the country’s Shiite-led government.

Attacks by al-Qaeda and other Sunni extremists against Shiites have mounted in Iraq this summer, even as Syria’s mostly Sunni rebels battle a regime dominated by the Alawite sect, which is an offshoot of Shiism. Waves of attacks on July 23 and Aug. 16 in Baghdad and other cities killed scores. The offensive seems to be pushing Iraqi Prime Minister Nouri al-Maliki, a Shiite, to join with Iran in backing the Syrian regime.

After preventing Iran, at the request of the United States, from flying supplies through Iraqi airspace to Syria this year, Mr. Maliki allowed the flights to resume in July. The Obama administration, having failed to conclude an agreement last year that would have maintained a small U.S. military force in Iraq, now lacks the leverage to prevent this sectarian polarization or to help combat al-Qaeda’s resurgence.

In Lebanon, several rounds of fighting have erupted between Alawites and Sunnis in the coastal city of Tripoli. Lebanese villages near the Syrian border, meanwhile, endure nearly nightly artillery barrages by the Syrian army, meant to prevent Sunni fighters and arms from crossing the frontier. In Turkey, sectarian tensions are rising between Alawites in the border province of Hatay and Sunni refugees from Syria — prompting the Turkish government to order the evacuation of refugees from the border region. Clashes between the Turkish army and Kurdish guerrillas also are increasing, encouraged in part by the Syrian regime’s ceding of territory to Syrian Kurds.

This complicated and sometimes obscure sectarian bloodletting may seem unrelated to American interests — but it is not. If the fighting continues to spread, important U.S. allies, including Saudi Arabia and Turkey, could be destabilized; both are indirectly backing Sunni fighters. The fragile political order in Iraq, bought with thousands of American lives, could collapse. Al-Qaeda could acquire new recruits and sanctuaries across the region.

The best means of preventing this, as State Department Middle East experts have been pointing out for months, is to accelerate the downfall of the Syrian regime. There are several ways of doing that, short of direct military intervention: materiel aid to the rebels is one. Now that its refusal to take that step has led to the very consequences it warned of, the administration would be wise to reconsider.