Mr. Laser Beam wrote:

It has been suggested that, had Picard and crew not appeared in the past of 2063 as shown in ST:FC, that the Mirror Universe resulted from it.
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I'm not so sure seeing as in a Mirror Darkly had what looked like Armstrong planting a Terran Empire flag on the Moon, which would mean the Terran Empire would be around at least since the 1960s in the Mirror Universe.
Come to think about it that would only be about 30 years after certain time travel incident that would have a major effect on the outcome of WWII, makes you wonder doesn't it.