BEIRUT, Lebanon — Syria is falling apart, as warring groups carve it into de facto fiefs, and it is increasingly hard to see how its disintegration can be reversed.

Within the country, no single group can muster enough territory or popularity to win an increasingly fragmented war — or even to negotiate and enforce an eventual peace. From outside, regional and global powers tug at the country in a proxy fight for their own interests, but none of them have the influence or, apparently, the will to reunite Syria.

In the crucible of war, as the Middle East undergoes its broadest reshaping since the collapse of the Ottoman Empire in the early 20th century, de facto spheres of influence and control are crystallizing, even as official borders remain unchanged.

The militant group Islamic State, also called ISIS or ISIL, is solidifying its rule in Syria’s eastern half, alongside its territory in neighboring Iraq. The local affiliate of Al Qaeda, the Nusra Front, and other insurgent groups — mostly Islamist, some relatively secular — are staking out territory in the northwest and the south.