The 717 campaign was the Umayyad Caliphate's major combined armed offensive against the Roman Empire, and one of the largest joint sea and land military operations across the entire medieval period. It was extensively funded and prepared by the Caliph himself. The expedition came about as a result of an extended tug of war over Anatolia between Constantinople and Damascus. By occupying Constantinople, Caliph Sulayman hoped to take advantage of Roman instability (see the 20 years anarchy) in order to completely shatter its hold over Anatolia in a single campaign and thus solve the entire Anatolian question in one fell swoop. Had the Caliph's armies under the command of Maslama ibn Abd-Al Malik been victorious, it is quite likely that the Umayyads would have held onto the city, because the only way to effectively besiege it would be to gain naval superiority and blockade it, which no other medieval power during this period other than the Romans and the Umayyads were able to project into the Aegean.
What would the effects of such a victory have been? It's hard to determine without relying on pure counterfactuals. Given that the goal of the campaign was to open up a vacuum in Anatolia ripe for Umayyad domination, large swathes of the region (if not all of it) would gradually be brought under Umayyad/Arab administration or vassalization. Certain outposts may have remained such as Trebizond but it's hard to determine how long for. Muslim rulers constantly competed with the Romans for control over the Mediterranean Islands as well, such as Crete, Cyprus, Rhodes and Sicily. The former three exchanging hands multiple times between them. However, without a Roman Navy, it seems very plausible that these islands would have permanently come under Arab rule, and would serve as strategic launch pads for further raids. A notable example of this in our timeline being Chandax, Crete, which was conquered by Andalusian exiles before being reconquered a century later by Byzantium.
On a wider political level, the Roman Empire would in a very real sense, cease to exist. Without Constantinople as its central authority, various neo-Byzantine rump states across Greece and Italy, some of which perhaps claiming to be a legitimate successor of the empire could emerge instead. When it comes to religious and cultural changes, the most noticeable impact would of course be in Anatolia, where I predict an even greater fusion of Greek and Arab ideas/culture/traditions than in our timeline would occur. An ethnically greek population adhering to Islam could likely emerge, which whilst becoming muslim, would maintain its greek language, traditions and customs. Similar to what happened to Iran and most other countries with sizeable muslim populations today, where whilst large swathes of them were brought under Muslim rule for many centuries, they still retained their heritage, culture, languages and traditions irrespective of any religious changes. Further fusions could include the adoption of Roman law alongside the shari'a to govern its greek population; comedies, tragedies and oratory arts (an epic tragedy of the Martyrdom of Hussein could be interesting); Roman-style civil service governance in caliphates and emirates, and Caliphs occasionally employing Byzantine Emperor's trappings as divine legitimacy and legal intervention (pardoning the guilty); monastic-style orders of Islamic mystics developing earlier than OTL. An even greater exchange of philosophical and scientific ideas between Muslim and Christian Europe may occur than in our middle ages, perhaps leading to socio-political circumstances that kickstart an earlier or alternative Renaissance-like transformation period more explicitly influenced by Islam and various near eastern intellectual environments.
An Arab conquest of the Balkans and Eastern Europe following this victory is hard to predict and I would argue unlikely given the difficulties Byzantine Constantinople experienced in confronting the Bulgarians and maintaining control over the region. However, without the central authority of the Roman Empire and its influence in the spread of Orthodox Christianity, it is quite likely that throughout the Balkans and across the Black Sea in modern Russia, the rulers, nobility and merchants may gradually adopt Islam instead. The spread of Islam through trade followed by the conversion of nobility and local rulers is also how Islam gained a foothold in many other parts of the world, such as Indonesia, which today houses the largest muslim population globally. Previously pagan empires, city states and local rulers across eastern Europe may well eventually choose to recognise the superiority of the religion of the Greco-Roman-Arab civilisation centred in Constantinople, converting to Islam instead. It wasn't until the peak of Ottoman power in the 17th century, where the Balkans gained a Christian-Muslim plurality, later to be erased by the 19th/20th century nascent balkan nation states, which oversaw the systematic expulsion of their Muslim populations, coterminous with the genocides of christian minorities of Anatolia under the CUP dominated Ottoman government.
Therefore, it is quite likely that many of these regions would gradually develop a Muslim plurality or majority throughout the medieval and early modern period, similar to the Malay Archipelago. Furthermore, assuming national identities develop across Europe as they did in our timeline, it is also highly plausible that Islam, or at least a coexistence between Islam and Christianity becomes ingrained within the values/identities of these various countries. Perhaps early 20th century Russia and Austria-Hungary would be composed of Muslim-Christian pluralities-assuming the massive transnational polities that emerged in our timeline aren't butterflied away (they probably would be, imagine a Muslim USSR lmao).
