Stefan Milo is also great :)

Friendly reminder to read the articles posted, bc it answers your question within the first couple of sentences. Also, it’s extremely short.

Before reading the article, I was assuming the Arab invasion of the Eastern Roman Empire or the Roman/Byzantine reconquest of their eastern and southern territories from the Persians were most likely. After reading the article, it seems experts have pegged the Arab invasion as the likeliest explanation, which makes sense.

Decades of war weakened the Byzantines and the Sassanid Persians, allowing the previously relatively minor Arab tribes to conquer the entirety of the Persian Empire and much of the Eastern Roman Empire (although, the Roman/Byzantine state would survive for another 800 years til 1453).