"I don't want it to happen," he writes, but then adds that it "probably will happen."
Trump further calls tonight's deadline, which he himself set for Iran, one of the "most important moments in world history."
The president also describes Iran as having undergone a regime change, where "smarter and less radicalized" people have taken over, and that something revolutionary and "wonderful" can happen.
"We will find out tonight," he writes.
The deadline is about Trump wanting to make a deal with Iran; if one is not reached, he has threatened to bomb the country "back to the Stone Age."
At the same time, reports are coming from Iranian state media about explosions on the important oil island of Kharg; this information has not yet been officially confirmed.
