Yes, this is 5th-6th century in date. Had the Byzantines done a better job at cultivating allies, particularly the Armenia which gave them breathing space as a buffer state, instead of undermining it and sacking their capital, the Roman Empire might have lasted longer than 1453.
Yeah, it also didn’t help when the Crusaders sacked Constantinople and set up the Latin Empire in 1261... The subsequent fighting between the various Crusader fiefdoms and the Nicaean state took fifty years and led to a crumbling of defenses against the Turks.