Update (1030ET): China has weighed in again on the tensions between the US and Iran, and its position is clear: If violence breaks out, it's solely the responsibility of the US.
Even without violating Iran's airspace, the US drone must have flown very close to it. Trump administration abruptly changed Iran policy, leading to tensions in the Persian Gulf. If war breaks out there in the end, Washington will be primarily responsible for it.— Hu Xijin ??? (@HuXijin_GT) June 21, 2019
Update (0944ET): Citing sources familiar with the administration's decision-making, Vox reports that the US military could go ahead with the strike on Friday.
"Two US officials and one person familiar with deliberations tell me that there are still plans for striking Iran on Friday night, but it's possible that plan won't be put into action."
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Update (0905ET): President Trump has tweeted, in an extreme show of transparency, why he decided against airstrikes on Iran:
"President Obama made a desperate and terrible deal with Iran - Gave them 150 Billion Dollars plus I.8 Billion Dollars in CASH! Iran was in big trouble and he bailed them out. Gave them a free path to Nuclear Weapons, and SOON. Instead of saying thank you, Iran yelled 'Death to America'.
I terminated deal, which was not even ratified by Congress, and imposed strong sanctions. They are a much weakened nation today than at the beginning of my Presidency, when they were causing major problems throughout the Middle East. Now they are Bust!
On Monday they shot down an unmanned drone flying in International Waters. We were cocked & loaded to retaliate last night on 3 different sights when I asked, how many will die. 150 people, sir, was the answer from a General.
10 minutes before the strike I stopped it, not proportionate to shooting down an unmanned drone.
I am in no hurry, our Military is rebuilt, new, and ready to go, by far the best in the world. Sanctions are biting & more added last night. Iran can NEVER have Nuclear Weapons, not against the USA, and not against the WORLD!"