WASHINGTON • There is a basic fact about Syria's civil war that never seems to change: It frustrates any attempt at resolution.
Despite many offensives, peace conferences and foreign interventions, including this week's Turkish incursion into a border town, the only needle that ever seems to move is the one measuring the suffering of Syrians - which only worsens.
Academic research on civil wars, taken together, reveals why. The average such conflict now lasts about a decade, twice as long as Syria's so far. But there are a handful of factors that can make them longer, more violent and harder to stop. Virtually all are present in Syria.
Many stem from foreign interventions that were intended to end the war, but have instead entrenched it in a stalemate in which violence is self-reinforcing and the normal avenues for peace are all closed. The fact that the underlying battle is multi-party rather than two-sided also works against resolution.
When asked what other conflicts through history had similar dynamics, Professor Barbara Walter of the University of San Diego, a leading expert on civil wars, paused, considered a few possibilities, then gave up. There were none. "This is a really, really tough case," she said.
Most civil wars end when one side loses. Either it is defeated militarily, or it exhausts its weapons or loses popular support and has to give up. About a quarter of civil wars end in a peace deal, often because both sides are exhausted.
That might have happened in Syria. The core combatants - the government and the insurgents who began fighting it in 2011 - are quite weak and, on their own, cannot sustain the fight for long.
But they are not on their own. Each side is backed by foreign powers - including the United States, Russia, Iran, Saudi Arabia and now Turkey - whose interventions have suspended the usual laws of nature.
This is why, according to Mr James Fearon, a Stanford professor who studies civil wars, multiple studies have found that "if you have outside intervention on both sides, duration is significantly greater".
Stalemate is also driven by uncertainty. No one is sure what a postwar Syria would look like or how to get there, but everyone can imagine a worse situation. This creates a status quo bias, in which combatants are more worried about preserving what they have than risking it to pursue their broader goals.
The only certain way to break the logjam is for one side to surge beyond what the other can match. Because Syria has sucked in two of the world's leading military powers, Russia and the US, that bar most likely could only be cleared by a full-scale invasion.
Peace deals often succeed or fail on the question of who will control military and security forces. In Syria, this may be a question without an answer. After a war as brutal as Syria's, in which more than 400,000 people have been killed, the combatants reasonably fear they will be massacred if the other secures too much power.
According to a 2015 paper by Prof Walter and Professor Kenneth Pollack, a Middle East expert, "outright military victory in a civil war often comes at the price of horrific (even genocidal) levels of violence against the defeated, including their civilian populations".
This could bring entirely new conflicts to the Middle East, they found: "Victorious groups in a civil war sometimes also try to employ their newfound strength against neighbouring states, resulting in interstate wars."
This is not a drift that anyone wants, but it is the direction in which Syria's many domestic and foreign participants are pulling the country, whose darkest days may still be ahead.
NEW YORK TIMES