What if the Umayyads had won the Siege of Constantinople (674-678 AD), defeating the Byzantine navy and looting and completely occupying Constantinople?
Would this pincer movement have spelled the beginning of the end for a free and independent Christendom? Would the Umayyad's have had a chance to not only conquer all the Byzantine Empire after this, but also move into the Balkans like the Ottomans did centuries later?
How far to the north can the Muslim conquest of Europe go?
Could it be that the second Islamic civil war would be ruled out?