“It takes time to build trust,” said Paul Eaton, a retired major general and veteran of the Iraq war. “And any time you erode trust, like this, it’s that much harder to bring it back.”

At the end of the Persian Gulf war nearly 30 years ago, the U.S. allowed Saddam Hussein to crush insurgents in Iraq, including Kurdish groups, leaving him in power until the U.S. invaded more than a decade later.

Catch up: At least 16 Kurds were reported to have been killed in Turkey’s assault in Syria, a monitoring group said today. The attack on the Kurds, a crucial American ally in the fight against the Islamic State, came days after President Trump agreed to let the operation proceed. Here’s a quick look at the history behind the conflict.

What’s next: It’s too soon to say what will happen, but Pentagon officials express fear that Turkey’s incursion into Syria could lead to the release of tens of thousands of ISIS fighters and the return of the self-proclaimed caliphate that the U.S. and its partners have spent the past five years destroying.

Related: The American military was working to remove as many as several dozen Islamic State detainees from Syria. The U.S. already has two British men in custody who tortured and killed Western hostages, according to officials.