I think the conquest of Constantinople could prevent the fall of the Umayyads.
OTL, the failure of Constantinople led to the loss of 100,000 men and most of the Umayyad fleet. As well as a loss of prestige/legitimacy, due to losing the mandate of heaven due to their military failures.
Umar II then went on a more defensive approach. Scared the Byzantines would attack Syria itself, withdrawing troops from Anatolia and considering abandoning Sindh and Andalus entirely to improve defences of the Caliphal centre.
After his death, his successors undid this, and pushed every frontier simultaneously. Despite many of them bringing in no wealth and were very difficult campaigns, such as Khazaria, Central Asia, Afghanistan. And rich frontiers like India being given only 15,000 men....
In Hisham's reign there were significant loses on almost every frontier: the Khazars took Ardabil the Azerbaijani capital, the Turgesh Turks defeated the Muslims on day of thirst and day of the defile Battle of Poitiers was lost, Byzantines defeated the Muslims at Akroinon and All of Gujarat, Punjab and even Sindh was lost etc. Overall this resulting in at least 106,000 troops dying just 2 decades after failure at Constaninople. (End of the Jihad state table 4)
All these losses resulted in the Syrian army, the backbone of the Umayyads being sent to reinforce the provinces. This scattered them throughout the Caliphate, making Syria vulnerable. It also increased provincial tensions who didn't like the Syrian troops.
Worst of all was the Great Berber Revolt, wherein almost the entire Maghreb was lost and tens of thousands of Syrian troops died.
By Hisham's death, the Syrians were too weak to continue their forced hegemony over the Caliphate. Meanwhile, Walid ii pivoted too much towards the Jazirans, resulting in the Syrians assassinating him. Leading to an Umayyad civil war in which Marwan ii and the Jazirans won, afterwhich the Syrians no longer played an important role.
But during the Umayyad civil war, the rest of the Caliphate revolted. Marwan spent the rest of his reign desperately putting down Alid and Kharijite revolts (some anti Marwan Umayyad princes even joined forces with the Kharijites and acknowledged a Kharijite, non Qurayshi Caliph). Until the Abbasids reared their head at the end. Though the Abbasids were almost defeated at the Battle of the Zab....
ITTL, 100,000 troops aren't lost, neither is the costly Umayyad navy. Instead immense wealth is gained via the conquest of the imperial capital of the Romans.
Beyond that, Umayyad prestige and legitimacy would skyrocket. Since there are Hadeeth about the conquest of Constantinople which they could use as propaganda.
Umar II in particular would receive even greater acclaim than OTL [only Umayyad not totally despised by Shi'ites or Khawarij]. With the populace seeing his personal piety as having pleased God, thereby allowing the successful conquest of Constantinople.
(Perhaps he could even marrying into the Alids....)
Ideally this would
prevent his assassination. Perhaps allowing him to rule over 20 years longer into his 60s (740s) this alone would prevent the Abbasid revolution, due to his popularity and favourable policies towards the non-Arab converts.
Consolidating Anatolia
Even despite the victory at Constaninople, Ideally Umar II would still end the policy of expansion on every frontier simultaneously.
- Particularly making the Caucasian frontier defensive. Moving the garrison to Derbent and Tbilisi/Darial instead of distant Ardabil. No longer attempting to campaign into the Khazar steppes.
- Similar for Transoxiana. Consolidating control over easier to control Chaghaniyan/Tukharistan and Khwarezm. But not attempting to cross the Oxus into Soghdia or Ferghana, where the local princes called upon the Turgesh or Chinese to help them against the Arabs.
- Completely stopping all campaigns Zabulistan and Kabul, since little is gained.
This frees up manpower for the only 2 remaining active frontiers: Byzantine wars and the Lucrative Indian invasions. And prevents losing tens of thousands men for almost no gain which antagonised the provincial garrisons against the Syrians, who had the easier and more lucrative Anatolian frontier
Beyond that, Umar ii's comparative popularity among the Shi'ites and Khawarij should allow him to remilitarise the 100,000 or so Iraqis who Hajjaj demilitarised in 701 in wake of the revolt of the Ibn al Ashath. Integrating the Iraqis, making the Umayyad powerbase more broad based. Instead of being Syrian supremacist to the exclusion of the Iraqis.
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Most importantly trying to integrate the Berbers far more effectively, thereby reducing manpower problems as hundreds of thousands of Berbers are included in the army. (OTL 300,000 may have partaken in the great Berber revolt)
As OTL, Umar would remove the Jizya from the Berber Muslims. But going even further by perhaps appointing some Berber governors of Maghreb Aqsa and Awsat (Morocco and Algeria) and perhaps even Ifriqiyah and Andalus. Making them much less hostile to the Umayyad regime.
Similarly sending scholars and theologians to curtail the spread of anti Umayyad Kharijite ideology among the Berbers.
Providing important manpower for the invasion of Italy and western Mediterranean navies.
The capital wouldn't be moved to Constaninople. At least for quite a few decades.
The main reason for this is that
Iraq is the Bankroller of the Caliphate, producing around a fifth of total revenues and 4x more than Egypt. The conquest of Constantinople wouldn't change that too drastically.
But Constaninople is too distant from Iraq to control it effectively. And the move would encourage the anti Umayyad Iraqis to revolt. Cutting off the whole east from the Caliphate.
Instead, the capital could be moved to
Antioch, which can use its Mediterranean navy to better control Constaninople. But also being in Syria, the home of the Umayyad armies, able to control Iraq and the east.
So Constaninople would be governed by Maslama ibn AbdalMalik, leader of the 2nd siege and Umar ii's cousin till his death in 738.
The most immediate concern for the Caliphate is to strengthen its control over Anatolia, allowing land connection to Constaninople.
The Anatolian plateau is difficult to control, and is a difficult climate for the Arabs. On the otherhand, the coastal regions are much more familiar climates to the Arabs, which is one of the reasons why Cilicia was able to be settled so effectively by the Arabs.
So the centres of Umayyad control would be on the coast:
- Control of Silfke allows control of the Servatul pass, which allows control of Karaman/Iconium - a central location on the Plateau.
- While the Antalya and the Pamphylian valley would serve as a second Cilicia and major centre of Arab settlement to control Phrygia.
- And the wealthy west Anatolian/Lydian coastal river valleys and Bithynian/Mysian coast would similarly be major sites of Arab settlement.
From the coastal plains, the Muslims would exert control on central Anatolia. Which would probably retain a large number of their pre conquest lords, whom the Arabs have made treaties with, particularly the very mountainous Paphlagonia and Pontus regions. Similar to the situation in Arab Armenia and the Bagratids. Or the initial conquest of the Iranian plateau. Or the treaty with Theodemir of Murcia in 713.
Potentially pastoralist Arab nomads, could be settled in the steppes of central Anatolia to increase arabisation and Umayyad control. Though Anatolia's different climate would require them to use cold tolerant Bactrians or Hybrid camels instead of dromedaries. So this might not be effective....
Much of the flat regions of Thrace near Constaninople would fall relatively easily. Taking a defensive approach against the Bulgarians by defending the passes of the Hameaus/Balkan mountains.
While Thessaloniki and Athens would be vulnerable to Umayyad Aegean fleets. But beyond the coast and plains, the Arabs would have little to no control.
Conquest of Italy
(
OTL, the Muslims managed to reach Tours and Sens. Take Balearics and Sardinia and besiege Syracuse. All Without Constaninople.
In a TL with Constaninople they should be able to achieve far more....)
Leaving the Byzantine remnants in Italy. But they only control Ravenna, Rome, Calabria, Apulia and Sicily. With the rest being in the hands of the Lombards.
The first few years of the Byzantines in Italy would probably be incredibly divided with various candidates for emperor all fighting one another for the small resources remaining. In which some candidates would make pacts and alkiances with the Lombards.
Since Liutprand is the strongest force in the peninsula, I see his chosen Byzantine candidate as winning with his support. Unifying all Italy under a Lombard-Byzantine alliance.
The shift of the Byzantines to Italy would make Iberia and Maghreb far more strategically important to the Umayyads. So unlike OTL, they wouldn't be neglected, anarchic backwaters.
Instead, Umayyad princes themselves would move to the Maghreb and Andalus to take charge of the new theatre of the Byzantine wars. Along with tens of thousands more troops.
These Umayyad princes would become the governors, ending the chaos of Andalus, where governors were constantly being switched. Bringing stability to the province.
While their personal leading of campaigns side-by-side the Berbers should develop a sort of esprit de corps with them. Making the Berbers more loyal to the Umayyad house, instead of seeing them as distant and foreign occupiers.
The most important thing is to develop the Umayyad navy of the western Mediterranean, to challenge the Byzantines.
Ifriqiyah already had sizeable fleets. Beginning its annual attacks on Sardinia, Sicily or Balearics from 704 onwards.
But the central and western Maghreb (Algeria and Morocco) as well as Iberia didn't have a navy at all (Tangier only had ferrying ships, not warships). While the eastern Mediterranean has largely been pacified with the conquest of Constaninople. Allowing much of the Levantine and Egyptian fleets to be shifted westwards.
Founding Bejaia, Algiers, Oran, Rabat, Casablanca as new naval bases. In the process increasing the development and integration/consolidation of the western Maghreb. Helping to prevent revolts.
While in Iberia, revitalising all ports of the peninsula: Heulva, Cadiz, Malaga, Cartegana, Valencia, Tortosa, Barcelona and Narbonne. And to a much lesser extent the Atlantic ports too: Lisbon, Coimbra, Porto, Coruña, Gijon, Santander and Bilbao. These naval bases on the Asturias coast, would be very difficult for the Asturians to attempt to conquer, due to their lack of a navy. Largely hemming them in the mountains, preventing the reoconquista.
This Atlantic navy would be able to raid western France. Making the invasion of Aquitaine much more successful than OTL, taking Bordeaux and going up the Garronne to assist in the 721 siege of Toulouse.
(Perhaps allying with anti Frankish
Basques in the process, for the added manpower....???)
Then up the Loire to at least Angers and perhaps Orleans? Coming dangerously close to Paris....
Allying with the Bretons and even Saxons against the Franks.
While from Constaninople supporting the Avars to attack the Franks from the east, as well as attack the Bulgars.
A large navy at Narbonne better control Septimania and push to Marseille and the Provence coast. Perhaps as far as Genoa... It would also allow for raids up the Rhone in tandem with the land forces for much more effective campaigning.
But most of this naval expansion would be for the purpose of attacking the Byzantines of Italy.
Back in 707, Musa ibn Nusayr defeated the Byzantines governors of Balearics, exacting a tribute out of the islands.
Then in 708, he seems to have conquered most of Sardinia in 708. Much of it was lost a few years later, but Cagliari was in Muslim hands until 732, since Luitprand negotiated with Muslims of Sardinia to buy Saint Augustine's bones. ('Sea of the Caliphs' pg 202).
Raids were sent against Sicily also. Between 704 and 740 there were at least 11 raids on the island. In 740 tribute was exacted from Syracuse but the Berber revolt forced them to leave before anything permanent could be achieved.
So in 719 and 720 the new ports and navies of the Maghreb and Andalus would strengthen their holds on the Balearics and Sardinia, directly controlling them and settling a few thousand Muslims.
As well as taking Malta, Pantelleria and perhaps the Aegadian and Aeolian islands too. In preparation of an invasion of Sicily.
At the beginning of 721's campaigning season, landing at Mazara of the Island, with at least 25,000 men (Aghlabids had less than 10k). But the difficult topography would make conquest slow.
From Mazara, marching on Palermo, besieging it for about a year, until it falls in 722. Then moving on Enna, a centrally located fortress in 723, falling after a few months towards the end of the year. Before in 724 moving on Syracuse. Falling after almost 2 years siege in early 726.
Only Messina would remain, falling in late 727. By 728, after 7 years, the entire island would've fallen.
The loss of the wealth and resources of Sicily would be a major blow to the ever dwindling Byzantines. And would open up the mainland to attack.
Meanwhile, by this time, the entire Aegean would've been conquered. As well as all Aquitaine and the Rhone valley up to Lyon.
From there, being the invasion of the mainland. The Aegean navies establishing coastal bases in along the Dalmatian coast, to assist the Sicilian navies in invading the Adriatic coast (perhaps enticing some Slavs to convert to join in on the lucrative raids...). Apulia being quite flat, making it not that difficult to conquer.
In contrast Calabria is very mountainous, so Muslim control would only be over a few fortified coastal cities. The main push would be towards Naples.
Taking Salerno after a few months siege in 733. Then moving on Naples the following year, falling after a year long siege in late 735. Leaving the road to Rome open.
Meanwhile in France, the much larger Muslim forces aided by the Mediterranean and Atlantic navies would win a decisive victory against Charles Martel in 732 Poitiers, sending him fleeing to Austrasia.
In 734 establishing Orleans as their northern base of operations, and beginning forays deep into Neustria. Particularly nearby Paris...
In 739, the Siege of Rome would begin falling a year later in 740. And with it the Roman empire.
Liutprand would retreat to his Capital Pavia, to protect the Po valley from further Muslim incursions and Po river raids.
But after his death in 744 and the weaker rule of Hildeprand and Ratchis, much of the Po valley would fall.
Meanwhile, in France, Paris would fall in 738. Pushing into Austrasia in 739. Besieging it's capital, Metz in 740. With the siege finishing shortly after the death of Charles Martel in 741.
So by the massive naval build up in the western Mediterranean, more effective integration and recruitment of the Berbers, converting unrewarding active frontiers to defensive and the popular Umar II ruling an extra 20 years, Europe would fall.
In 744, at the age of 63, Umar II would pass away, being the Caliph who not only conquered Constaninople, but Rome too.
As to whom his successor would be is unknown. Since Yazid ibn Abdulmalik, Hisham and Maslama had already passed. Some minor sons of Abdulmalik still live, so they could be chosen.
Or perhaps Umar's son himself.
Throughout these decades of campaigning, the Berbers would've heavily integrated themselves into the Umayyads regime. And would make up the majority of the navy and land army of all these western conquests.
So this new Caliph couldn't try to change Umar ii's entrenched policies even if he tried.
Similarly, the Iraqis would be remilitarised and engaging in some of the Westerly campaigns, being far more integrated also. The defensive approach on the Transoxianan campaign would mean the Khurasanis aren't devastated by the Turgesh. So they are happier with the regime.
Perhaps Jafar asSadiq could be a chief judge of Umar II, since he taught both Malik and Abu Hanifa. This would appease the Alids too....
By that Umar II would've created a much more broadly based regime, replacing the previous Syrian supremacist model.
This would allow the Umayyad Caliphate to continue, as long as it remains broadly based.