The outrageous and indefensible attack against the U.S. Consulate in Benghazi, Libya, that killed U.S. Ambassador Chris Stevens and three other American diplomats on Wednesday sends a powerful message about the treacherous currents blocking peaceful change in the Middle East.

Extremist-led attempts to prevent the emergence of modern societies in Islamic countries will continue to imperil U.S. efforts to improve conditions in that part of the world and the safety of diplomats like Ambassador Stevens, who risk their lives daily on the front lines of American foreign policy. But the events in Libya and Egypt should not deter U.S. efforts to promote the U.S. national interest in the region peacefully but forcefully. The last thing the United States can afford to do — what its enemies would love for it to do — is to abandon the strategy of creating stronger bonds with the emerging governments of the region in the wake of the Arab Spring.

This is not Tehran, 1979, when government-backed thugs sacked the embassy and took American diplomats hostage. The Libyan government was quick to issue an apology and to condemn the attacks, and its deputy ambassador to the United Nations called Ambassador Stevens “one of the greatest friends of Libya.”

Amid the violent protest, Libyan troops fought back against the mob, helped protect U.S. diplomats and took Mr. Stevens’ body to the hospital, where he later died of smoke inhalation. Libya’s government vowed to bring the perpetrators to justice, and President Obama pledged to offer U.S. assistance to make that happen.

It will take time to sort out the events in Benghazi, but U.S. intelligence officials said the attack was too well-coordinated, well-planned and well-armed to have been the work of a spontaneous mob enraged over an amateurish film allegedly made in the United States that disparaged the prophet Mohammed. It is surely no coincidence that the attack, along with an attempt to breach the U.S. Embassy compound in Cairo by another mob, occurred on the 11th anniversary of the 9/11 attacks.

This underlines the continuing dangers that United States and pro-democracy movements confront as determined adversaries resort to violence to turn back the clock and impose ruthless tyrannies in various countries.

It also emphasizes the need for U.S. resolve in the face of the challenge. The United States is under attack precisely because it is seen as an ally of the forces of peaceful change. It must not allow incidents like these to drive a wedge between U.S. policy and governments like the one in Egypt, caught between the conflicting currents of democracy and extremism.

Mr. Obama was right to reject “all efforts to denigrate the religious beliefs of others,” an implicit condemnation of provocations that inspire religious hatred, but he forcefully noted that there can be no justification for “brutal acts” like the incidents in Libya and Egypt.

Predictably, the attacks were quickly mired in political controversy at home. Candidate Mitt Romney jumped the gun in attacking Mr. Obama as an “apologist” because of a statement issued by the U.S. Embassy in Cairo before the assaults took place. Reacting to the death of U.S. diplomats by seeking to take political advantage is profoundly inappropriate.

This is a time for unity, as much as it was when Americans rallied behind President Bush 11 years ago. Wednesday’s events are reminders that the war that began on 9/11 is far from over.