It is not as if the world has no evidence of Syria’s ordeal, which has killed an estimated 150,000 people. Syrians have issued a sustained, collective cry for help from what is now probably history’s most-documented manmade disaster. They capture appalling suffering on video and beam the images out to the world: skeletal infants, body parts pulled from the rubble of homes, faces stretched by despair, over and over.

Despite that, to the bitterness of Syrians, the world’s diplomatic attention is drifting. Even as Syria’s epic suffering is remaking the human geography of the Middle East and beyond, initiatives to ease the crisis have sputtered and failed to offer effective help. Already tenuous hopes for an internationally brokered peace settlement have further faded as Russian-American relations worsen.

António Guterres, the head of the United Nations refugee agency, said that is in part because there is no obvious path to a coherent global response. Given the world’s growing unpredictability, and competing priorities, “crises are multiplying and more and more difficult to solve,” he said. “Afghanistan is not finished. Somalia is not finished. It’s overwhelming.”

All the while, Syria is falling apart.

Last weekend, another vital center of opposition life — the city of Yabrud, near the Lebanese border — fell to pro-government forces. As each such haven has been shattered, like Homs and Qusayr, it has become a watchword for civilian suffering, and more are displaced.

The country is threatened with de facto partition among the government, Kurdish militias and a patchwork of insurgent groups, some seeking to impose extremist Islamist rule. Criminal gangs profit from chaos, and pro-government militias increasingly threaten to slip from state control. A regional proxy war between Saudi Arabia and Iran further polarizes the conflict and fuels its sectarian dimension.