So I have been recently thinking on ways the Byzantines could've survived longer. One of them I have thought of is that Byzatines won the Byzantine-Sasanian War and actually invaded and took over the Sasanian Empire. This possibly could block the Rashdian Caliphate from central asia and them taking over Iran. Helping curb muslim influence. This could obviously could bring up the Byzantines spreading their influence into the steppes which could start a war against the Goturk Khaganate. What other ways could've the Byzantines lived longer.
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Perhaps The Byzantines looking back at their early Roman roots, had not relied on mercenaries, but on home grown troops. Citizen soldiers and duch.
That sounds like a very good solution, mercs are a voliatile resource.