Phocas
Phocas deeply mistrusted basically everyone especially the elites in Constantinople as he knew he was just a usurper and didn't have any connections with anyone. As soon as he became emperor, Sasanian Emperor Khosrow II used this as an excuse to invade the Eastern Roman Empire which at a time was facing troubles internally and elsewhere which would spark the beggining of a 2 decade long Roman-Persian War which would prove to be a disaster not only to the Persians but Also the Eastern Roman Empire.