In an internal memo, the diplomat, William Roebuck, questioned whether diplomacy, economic sanctions or increased military patrols might have deterred Turkey from taking over a region that was once controlled by America’s Syrian Kurdish allies. “We won’t know,” he wrote, “because we didn’t try.”

His memo appears to be the first formal dissent on Syria from a Trump administration official to be made public. (Pentagon officials were alarmed by the sudden shift in Syria policy, but top officials kept their views private.)

How we know: The Times obtained a copy of the unclassified memo from someone who said it was important to make Mr. Roebuck’s assessment public.

Quotable: “One day when the diplomatic history is written,” Mr. Roebuck wrote, “people will wonder what happened here and why officials didn’t do more to stop it.”

On the ground: Our reporters traveled along the only major road that runs the length of northeast Syria to make sense of the new power dynamic taking shape there. It wasn’t easy.