“I had not heard the word ‘genocide’ used in the Situation Room before,” the official said. “That word has a lot of weight.”

The reports from the intelligence community and the State Department were vivid and compelling, the official said: People were dying of hunger and thirst, women risked being enslaved and the existence of a religious minority looked imperiled. It more than met the legal definition of genocide, aides told Obama.

“While we have faced many difficult humanitarian challenges, this was in a different category,” the official said. “This was qualitatively different from even the awful things we have confronted in different parts of the region because of the targeted nature, the scale of it, the fact this is a whole people. That kind of shakes you up, gets your attention.”

After a 90-minute meeting Thursday morning, the president headed to Fort Belvoir to sign a veterans health bill, making no mention of the worsening conditions in northern Iraq or his private deliberations on what to do about it…

The major concern was the seizure of the Mosul Dam. If it were ever breached, the result would be catastrophic, causing the flooding of Baghdad and the Kurdish capital of Erbil, officials said.

“We were watching some metrics to have a sense of tipping points,” one official said. “And one of them was very much the dam.”