It’s difficult to say either or not there existed efforts to coordinate between Byzantine Emperor Heraclius and the Sassanians. There certainly did not exist an consolidated alliance or direct interactions from the respective monarchs in coordinating a collective defence. William Kaegi in "Byzantium and the Early Islamic Conquests” provides that:

Distance, differing languages, separate bureaucracies, outlooks, and interests all combined to make such coordinated strategy and operations very difficult to accomplish.

However, it’s equally difficult to say that no cooperation occurred whatsoever between the two states. Kaegi who states that it was most likely that Byzantium occupied portions of northern Iraq speculates that the proximity of those governors to the crisis in Persia would have evidently made them aware of the threat of the Arabs and their progress against the Sassanian defence effort. Thus, direct opportunities certainly would have presented themselves for Byzantine units stationed there to provide a measure of assistance, though it is ultimately unclear to us whether or not action was made towards those opportunities.

In one instance, which was the Battle of Firaz in 634 CE and taking place on the Euphrates boundary between the Byzantine Empire and Sassanian Iraq, it is stated that when Khalid ibn al-Walid struck at the last Sassanid garrison left stationed on the Euphrates, the Byzantines crossed the river to assist the Sassanids. The combined forces of 100,000 men were defeated by Khalid’s 20,000.

However, the reliability of the account of this battle has been questioned as the only sources appear to be those derived from accounts of Khalid centuries later. For example, Heraclius could not have provided a garrison along the Euphrates of such a scale so that the opposition against Khalid in Firaz numbered tenfold against the Arab force and it is possible the Byzantines at that point would have still seen the Arab incursion as a mere raid against Persian land. The idea of Firaz being a fabricated account however tempting is yet without sufficient evidence as is ultimately much of the topic of Byzantine-Sassanian cooperation.

At this point, lack of evidence devolves the discourse into speculation, but the skeptic camp however provides an interesting notion that a possible reasoning for a fabrication of a battle like Firaz was to impress the superiority of the the Arabs over a combined Byzantine-Sassanid army, it is there likely that further instances would be similarly documented for equal purposes.

Thus, if the Battle of Firaz did occur, and with the Byzantine supporting the Sassanians, it would be yet difficult to use this particular case reliably to infer support at the expanded possibility of similar small-scale coalition efforts that may have occurred.

———————————————————————————————————

Reg G. Grant. 1001 Battles That Changed the Course of World History, (New York: Universe Publishing, 2011), 108

Peter Crawford. The War of the Three Gods: Romans, Persians and the Rise of Islam, (South Yorkshire: Pen & Sword Books Ltd, 2013), 107

William Kaegi. Byzantium and the Early Islamic Conquests, (Cambridge: Cambridge University Press, 2005), 258-260