In OTL the Emperor Heraclius (from whose time historians
dub his Eastern Roman Empire "Byzantine" thereafter) was
personally leading his armies against the Sassanids (Persians
who had extended their empire to Syria) in 613 and survived
a defeat.

What if the defeat was more comprehensive and he died?

In OTL he fought a long war and despite further defeats
(Jerusalem fell to the Sassanids in 614,Alexandria in 619,
a siege of Constantinople was beaten off in 627) in the
end he prevailed and regained lost territory as the
Sassanids devolved into coup and counter-coup.

Without him--considered a critically capable emperor--the
Sassanids might have had the leisure to swoop south and
wipe out Islam in its formative years,establishing a
Zoroastrian beachhead on the Mediterranean instead of
being crushed by the rising Caliphate a few decades later.

-=-=-
The World Trade Center towers MUST rise again,
at least as tall as before...or terror has triumphed.