what if sassanids reinforced Byzantines in the middle of battle of Yarmuk as a friendly gesture to form an alliance

will this surprise attack win the battle?

will this cement an allaince and stop arabs ?
 
They can't. The Sassanid Empire was crumbling at the time, under pressure from other Arab armies and fragmenting into feudal states.
 
they really cant while Heraclius had made an allience with them they did not show up because they couldnt the plan was originally for Vahan to strike the muslims when the sassanids were ready as they can them thin them out Umar sent 6000 men from the persian front to the byzantine one the best one can hope for is the timing is good and rostram and Vahan attack at around the same time.
 
A better POD would be the Plague of Sheroe not killing Sheroe (Kavad II), which predictably threw the entire empire into chaos because the man had already killed all of his brothers. Even if the empire was bloodied in the 602-628 War, the Sassanids would (probably) not have collapsed if they did not sink into the 628-632 Civil War.

We could look to the Byzantine internal wars (Doukas-Romanos, de Bailleul's revolt, etc) after the 1071 Battle of Manzikert as a comparison. Manzikert itself was a lot less horrific than what came after.

Besides, a Roman-Iran alliance was already in place during the Rashidun Invasions. It just didn't accomplish its intended purpose.
 
We could look to the Byzantine internal wars (Doukas-Romanos, de Bailleul's revolt, etc) after the 1071 Battle of Manzikert as a comparison. Manzikert itself was a lot less horrific than what came after.
This. Both the Byzantines and the Sassanids had a tendency to defeat themselves via civil wars. My Professor in college once said that what Manzikert proved to the Seljuks that the Byzantines didn't unite after a hammering. They shattered.
 
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