While the Sasanians were running amok in Syria and Anatolia during their climatic final war against the Eastern Roman Empire, the Slavs (615) and later a combined force composed of them and the Avars (617) tried to capture Thessalonica, probably the empire's second most important city in the Balkans, the first obviously being Constantinople. Both of these attacks failed.
What if they were successful and Thessalonica was stormed and sacked? While I doubt this would do much to change the war against the Iranians and Heraclius' eventual victory (Khosrau II was an awful Shah, and the great initial conquests happened in spite of, not because of him) it would still be a substantial blow to Roman prestige and revenues.
Could the (likely Umayyad) Caliphate eventually capture Constantinople during one of its sieges of the city?
What if they were successful and Thessalonica was stormed and sacked? While I doubt this would do much to change the war against the Iranians and Heraclius' eventual victory (Khosrau II was an awful Shah, and the great initial conquests happened in spite of, not because of him) it would still be a substantial blow to Roman prestige and revenues.
Could the (likely Umayyad) Caliphate eventually capture Constantinople during one of its sieges of the city?