Hello,
I'll clarify this by saying quite simply as what point was the eventual fall of the Eastern Roman Empire a forgone conclusion? Was it when the Arabs invaded shortly after the peace between both the Sassanid's and ERE and they subsequently lost the Levant? Perhaps earlier? Or maybe even later than the Arabian invasion.
As always, thank you!
The Byzantine Empire survived long after the 7th century Arab invasions, not truly collapsing until final fall of Constantinople in 1453 (or, if you count Trebizond as a true heir to the Byzantine legacy, then it’s fall in 1461).
However, you are correct in noting that the Arab invasions fundamentally changed the Empire. Prior to those invasions, it stretched from Spain to Mespoptamia, including Egypt, the Levant, and even parts of Italy. The 6th and 7th centuries saw numerous invasions, not only from the Arabs, but also Slavic tribes such as the Avars and Bulgars that took over vast swathes of territory in the Balkans and displaced some of the native population. Pressure from those invasions in two directions, Slavs from the north and Arabs to the south, caused the Empire to contract, and focus around Greece and Anatolia.
Despite losing Egypt and other far-flung provinces, the Byzantine Empire actually enjoyed enormous success in its smaller form. Periods such as the 10th Macedonian renaissance and the 11th Komnenian restoration brought heightened power, culture, and presage to the empire that, although smaller than its 4th and 5th century form, continued to remain one of the dominant powers in the region.
If you want to find a precipice that marked the overall decline of the empire, I would point you to the 13th century, specifically the 4th crusade in 1204, during which Venetian and French knights sacked the city, dismantled the empire, and established short-lived petty crusader kingdoms in its former holdings. Although the empire was eventually reestablished almost 60 years later, it had lost enormous strength, and would never again grow to its former power. Many scholars, such as Edward Luttwak, actually point the 1204 as the date that marks the end of the Byzantine Empire, with the successive polities failing to earn the mantle of “Byzantine.”
For further reading, I highly recommend Edward Luttwak’s Grand Strategy of the Byzantine Empire. It has a mostly military focus, but also accurately describes the changes occurring throughout the Byzantine Empire’s history. Another great one is John Julius Norwich’s Short History of the Byzantine Empire, an abridged 3-volume work that is meticulously researched and well-written.