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President Trump described in more detail his decision to pull back strikes on Iran, saying he didn't feel an attack that left 150 dead was proportionate to the downing of a US drone.

"Nothing was greenlighted until the very end because things change," Trump said in an interview with NBC News. "We had something ready to go subject to my approval."

Trump said planes were not in the air when he ordered the pull back, but "would have been pretty soon."

"Things would have happened to a point where you would not turn back, you could not turn back," he said.

Trump said he asked his national security officials how many would be killed if he went forward, and was given an approximate figure of 150.

"I thought about it for a second and I said, you know what, they shot down an unmanned drone, plane, whatever you want to call it, and here we are sitting with a 150 dead people that would have taken place probably within a half an hour after I said go ahead, and I didn’t like it, I didn’t think, I didn’t think it was proportionate," Trump said.