Marwan was the most senior living Umayyad after Muawiyah, 25 years older than Yazid, and had a lifetime of experience in governance, unlike Yazid.
In his twenties he partook in the initial invasion of Ifriqiyah alongside Hassan, Husain, Ibn Zubayr, Ibn Umar and many other companions. Then he was appointed as governor of Fars, and then secretary to Caliph Uthman in Madinah, one the highest positions in the Caliphate.
Then under Muawiya he was given governorship over the Prophet's City, Madinah which had been capital of the Muslims for 35 years since 622 until 657, just 4 years prior to Marwan's appointment to the city. Being governor twice, 661–668 and 674–677.
At this stage, Madinah was the most influential city in the Caliphate, home to the majority of the companions and previous centre of government for 35 years. Only Kufa came close due to its large population and control over the enormous revenues of the Sawad.
But Damascus had only been capital for a few months on Marwan's appointment. And the city only had a small Muslim population and was still majority Christian.
In governing Madinah, Marwan governed over most of the still living companions including Hassan, Hussain and Ibn Zubayr, personally leading them in prayer, presiding over judicial issues and disputes (being a narrator of hadeeth, even in Bukhari and regarded as a faqeeh by some later jurists), and generally living among them. Thus he had a familiarity with Hussain and Ibn Zubayr, and they with him. Something Yazid completely lacked.
Though there were tensions, such as when Marwan protested Hassan being buried in the Prophet's house, since Uthman was prevented from that by the Khawarij. But he still partook in Hassan's funeral, carrying the bier and praising Hassan.
Or a brawl which seems to have occurred between Marwan and ibn Zubayr over insulting one another's father's.
Beyond that, the majority of the Umayyad house lived in Madinah, not Syria, since the Abi alAas branch of Marwan (and Uthman) was much larger than their Sufyanid cousins in Syria.
Madinah was also home to other influential tribes, who Marwan would've made ties and alliances with.
Finally, the OTL is testament to his excellent capabilities. Since after the near annihilation of the Umayyads following Yazid's death, he was elected by the Syrian tribal armies and remaining Umayyads as their leader at Jabiya, and went on to retake the rest of Syria and Egypt.
In the words of Bosworth: "Marwan was obviously a military leader and statesman of great skill and decisiveness amply endowed with the qualities of ḥilm (levelheadedness) and shrewdness, which characterised other outstanding members of the Umayyad clan".
Yazid had none of these traits. He was young with little to no experience, save his ceremonial role as leader of the siege of Constantinople.
He grew up in Syria not among the companions of Madinah, having little familiarity with them. And few connections to the Abi alAas branch or other influential non-Syrian tribes.
The one advantage Yazid does have, is his greater connections with the powerful Syrian Kalb tribe. But Marwan too did have a connection to the Banu Kalb via his wife Layla bint Zabban ibn al-Asbagh.
So ITTL, Muawiyah recognises that the only capable Umayyad candidate would be Marwan, not his own son.
Naming Marwan heir apparent in 676 - while Marwan is still governor of Madinah. Thus, he would be able to keep watch on Ibn Zubayr and Husayn.
On Muawiya's death he would need to move to Syria. But his lifetime and strong contacts within Hejaz should be able to keep better control over ibn Zubayr and Husayn.
If they do still rebel, then Marwan's familiarity with them would probably allow a preemption of what they were going to do. Fortifying Makkah and Kufa. Probably resulting in a less disastrous ending...
Since Karbala was the worst possible thing that could have happened in dealing with Husayn. And forever ruined the Umayyad reputation.
Ideally Marwan personally going to Kufa to negotiate with Husayn. Instead of leaving it in the hands of the brutal Ubaydullah ibn Ziyad, who Marwan didn't have that good of a relationship with, being the foremost Umayyad against Muawiya's adoption of his father Ziyad ibn Abihi into the clan.
OTL Husayn is said to have offered 3 options to the Umayyad army: He be allowed to return to Medina, Submit to Yazid directly, Or be sent to a border post where he would fight alongside the Muslim armies.
So ITTL, Marwan agrees and either send him back to Madinah. Or puts him on the Byzantine frontier - fromwhich he would be close to Damascus, and among the highly loyal Syrian army. So should be easier to control...
The lack of Karbala would be monumental for Umayyad public opinion. With Muslims in general and Kufa in particular not seeing them as pure evil.
Giving the Umayyads greater control over Iraq, the bankroller of the Caliphate. Allowing the large military populations of Kufa and Basra (probably around 100k in total) to remain militarised, alleviating some of the manpower problems the later Umayyads had.
And overall a more stable Caliphate, with future Alid revolts less likely due to a peaceful resolution with Husayn. If they still do occur, they would garner much less support, since Umayyads wouldn't be seen as that bad, making them easier to put down.
Prevention of Mukhtar's rebellion might mean Shi'ism doesn't even exist....
As for ibn Zubayr, then Makkah is isolated and easily defeatable. Though, the Marwan could probably prevent him from even reaching the city. Due to his influence in Madinah. And the traditional Umayyad influence in Makkah, since they have been the most dominant tribe of Quraysh since before Islam.
Thus the decade long civil war is prevented. Allowing continued expansion.
Most immediately in reinforcing Uqba ibn Nafi, to prevent the collapse of the Maghreb in wake of Battle of Vescera. Instead solidifying and consolidating his great raids through the establishment of Misrs in the central and western Maghreb. Namely Tiaret and Beni Melal. Then taking Carthage in the mid 680s. Completing the conquest of the Maghreb by around 690. Beginning raids on Sicily a few years later.
A Caliphate without a decade long civil war, would be able to take greater advantage of the 20 years anarchy.
In the east, Chaghaniyan/Tokharistan and Khurasan wouldn't fall to the Nezak Tarkhan of Hephthalites in 689. Instead expansion could continue into Khwarezm and Sogdia. Not pushing deeper into Fergana or Shash until the region has been consolidated.
Moving the capital to Amul/Turkmenabat. Since it's equidistant from Samarkand, Balkh, Merv and Kath. It's on the west bank of the Oxus, providing a barrier against the nomads and it is the major crossing point of the Oxus, making it even more defendable. The Oxus is Navigable from Ai-Khanoum to the Aral sea. Allowing the city to receive Khwarezm grain barges to become a populous and thriving metropolis. While it's Navigability could also be used to better control Khwarezm and Chaghaniyan, and for Watermill industry. Overall better in every way than Merv and Balkh.
In Sistan, instead of sending Yazid ibn Ziyad ibn Abihi to his death against the incredibly difficult terrain of Zabulistan (Afghanistan), sending him into the much easier and manyfold richer kingdom of Sindh. With assistance from a Basran naval force landing at Daybul. Taking Sindh by mid 680s. Bringing in immense wealth.
Allowing expansion into northern India, which was in a power vacuum and chaotic since Harsha's death over 3 decades prior.
Ideally, Marwan's lack of connection and attachment to Damascus would allow him to move the Capital to Raqqa/Barbalisos. Navigable via the Euphrates to Kufa and Basra, allowing much more control. With Iraqi grain barges sent upstream allowing a huge population, being one of the only locations for a Syrian metropolis which can rival and surpass Kufa - the largest city in the Caliphate and it's intellectual hub. Though hostile to the Umayyads. https://www.alternatehistory.com/forum/threads/raqqa-ideal-caliphal-capital.544649/
Overall a metropolis, navigable to Iraq, should allow a much more centralised Caliphate of than OTL Umayyads, who ruled out of small desert castles...
Raqqa being on the Syrian-Jazira border would also give the Qaysi tribes much more integration in the state, especially since Marj Rahit never occured.
This means less of a total Syrian-Yamani monopoly, which should lessen the later Yamani-Qaysi rivalry.
Though since Iraq isn't demilitarised and is far less hostile without Karbala, the Syrian monopoly wouldn't really exist... Making Umayyad rule much more popular among the non-Syrian masses
After a short reign of 5 years, his son AbdulMalik would become Caliph.
Able to enact his brilliant reforms immediately, without need of 7 years of difficult campaigning against Umayyad claimants and Zubayrids.
His centralising policies having a much greater efficacy from the strategic capital of Raqqa
During his 20 year reign, expansion would be mainly focused on the unfathomable riches of India. As well as the anarchy next-door in the Byzantine empire.....
In 692, Sisebert, the metropolitan archbishop of Toledo, and many Visigothic nobles rebelled against King Egica, taking the capital of Toledo and briefly installing Suniefred as King, minting coins in his name.
The Muslims across the straits would use the chaos to invade the fractured Visigothic kingdom, in a campaign similar to 711. But with support from the Maghrebi navy.
Taking all Iberia by 696/7. Leaving the door open to Frankia, which was reunifying and becoming stronger under Pepin of Herstal. But it's still very fractured and vulnerable...
Overall this is a much more popular and stable Umayyad Caliphate. That might be able to last indefinitely....
In his twenties he partook in the initial invasion of Ifriqiyah alongside Hassan, Husain, Ibn Zubayr, Ibn Umar and many other companions. Then he was appointed as governor of Fars, and then secretary to Caliph Uthman in Madinah, one the highest positions in the Caliphate.
Then under Muawiya he was given governorship over the Prophet's City, Madinah which had been capital of the Muslims for 35 years since 622 until 657, just 4 years prior to Marwan's appointment to the city. Being governor twice, 661–668 and 674–677.
At this stage, Madinah was the most influential city in the Caliphate, home to the majority of the companions and previous centre of government for 35 years. Only Kufa came close due to its large population and control over the enormous revenues of the Sawad.
But Damascus had only been capital for a few months on Marwan's appointment. And the city only had a small Muslim population and was still majority Christian.
In governing Madinah, Marwan governed over most of the still living companions including Hassan, Hussain and Ibn Zubayr, personally leading them in prayer, presiding over judicial issues and disputes (being a narrator of hadeeth, even in Bukhari and regarded as a faqeeh by some later jurists), and generally living among them. Thus he had a familiarity with Hussain and Ibn Zubayr, and they with him. Something Yazid completely lacked.
Though there were tensions, such as when Marwan protested Hassan being buried in the Prophet's house, since Uthman was prevented from that by the Khawarij. But he still partook in Hassan's funeral, carrying the bier and praising Hassan.
Or a brawl which seems to have occurred between Marwan and ibn Zubayr over insulting one another's father's.
Beyond that, the majority of the Umayyad house lived in Madinah, not Syria, since the Abi alAas branch of Marwan (and Uthman) was much larger than their Sufyanid cousins in Syria.
Madinah was also home to other influential tribes, who Marwan would've made ties and alliances with.
Finally, the OTL is testament to his excellent capabilities. Since after the near annihilation of the Umayyads following Yazid's death, he was elected by the Syrian tribal armies and remaining Umayyads as their leader at Jabiya, and went on to retake the rest of Syria and Egypt.
In the words of Bosworth: "Marwan was obviously a military leader and statesman of great skill and decisiveness amply endowed with the qualities of ḥilm (levelheadedness) and shrewdness, which characterised other outstanding members of the Umayyad clan".
Yazid had none of these traits. He was young with little to no experience, save his ceremonial role as leader of the siege of Constantinople.
He grew up in Syria not among the companions of Madinah, having little familiarity with them. And few connections to the Abi alAas branch or other influential non-Syrian tribes.
The one advantage Yazid does have, is his greater connections with the powerful Syrian Kalb tribe. But Marwan too did have a connection to the Banu Kalb via his wife Layla bint Zabban ibn al-Asbagh.
So ITTL, Muawiyah recognises that the only capable Umayyad candidate would be Marwan, not his own son.
Naming Marwan heir apparent in 676 - while Marwan is still governor of Madinah. Thus, he would be able to keep watch on Ibn Zubayr and Husayn.
On Muawiya's death he would need to move to Syria. But his lifetime and strong contacts within Hejaz should be able to keep better control over ibn Zubayr and Husayn.
If they do still rebel, then Marwan's familiarity with them would probably allow a preemption of what they were going to do. Fortifying Makkah and Kufa. Probably resulting in a less disastrous ending...
Since Karbala was the worst possible thing that could have happened in dealing with Husayn. And forever ruined the Umayyad reputation.
Ideally Marwan personally going to Kufa to negotiate with Husayn. Instead of leaving it in the hands of the brutal Ubaydullah ibn Ziyad, who Marwan didn't have that good of a relationship with, being the foremost Umayyad against Muawiya's adoption of his father Ziyad ibn Abihi into the clan.
OTL Husayn is said to have offered 3 options to the Umayyad army: He be allowed to return to Medina, Submit to Yazid directly, Or be sent to a border post where he would fight alongside the Muslim armies.
So ITTL, Marwan agrees and either send him back to Madinah. Or puts him on the Byzantine frontier - fromwhich he would be close to Damascus, and among the highly loyal Syrian army. So should be easier to control...
The lack of Karbala would be monumental for Umayyad public opinion. With Muslims in general and Kufa in particular not seeing them as pure evil.
Giving the Umayyads greater control over Iraq, the bankroller of the Caliphate. Allowing the large military populations of Kufa and Basra (probably around 100k in total) to remain militarised, alleviating some of the manpower problems the later Umayyads had.
And overall a more stable Caliphate, with future Alid revolts less likely due to a peaceful resolution with Husayn. If they still do occur, they would garner much less support, since Umayyads wouldn't be seen as that bad, making them easier to put down.
Prevention of Mukhtar's rebellion might mean Shi'ism doesn't even exist....
As for ibn Zubayr, then Makkah is isolated and easily defeatable. Though, the Marwan could probably prevent him from even reaching the city. Due to his influence in Madinah. And the traditional Umayyad influence in Makkah, since they have been the most dominant tribe of Quraysh since before Islam.
Aftermath:
Thus the decade long civil war is prevented. Allowing continued expansion.
Most immediately in reinforcing Uqba ibn Nafi, to prevent the collapse of the Maghreb in wake of Battle of Vescera. Instead solidifying and consolidating his great raids through the establishment of Misrs in the central and western Maghreb. Namely Tiaret and Beni Melal. Then taking Carthage in the mid 680s. Completing the conquest of the Maghreb by around 690. Beginning raids on Sicily a few years later.
A Caliphate without a decade long civil war, would be able to take greater advantage of the 20 years anarchy.
In the east, Chaghaniyan/Tokharistan and Khurasan wouldn't fall to the Nezak Tarkhan of Hephthalites in 689. Instead expansion could continue into Khwarezm and Sogdia. Not pushing deeper into Fergana or Shash until the region has been consolidated.
Moving the capital to Amul/Turkmenabat. Since it's equidistant from Samarkand, Balkh, Merv and Kath. It's on the west bank of the Oxus, providing a barrier against the nomads and it is the major crossing point of the Oxus, making it even more defendable. The Oxus is Navigable from Ai-Khanoum to the Aral sea. Allowing the city to receive Khwarezm grain barges to become a populous and thriving metropolis. While it's Navigability could also be used to better control Khwarezm and Chaghaniyan, and for Watermill industry. Overall better in every way than Merv and Balkh.
In Sistan, instead of sending Yazid ibn Ziyad ibn Abihi to his death against the incredibly difficult terrain of Zabulistan (Afghanistan), sending him into the much easier and manyfold richer kingdom of Sindh. With assistance from a Basran naval force landing at Daybul. Taking Sindh by mid 680s. Bringing in immense wealth.
Allowing expansion into northern India, which was in a power vacuum and chaotic since Harsha's death over 3 decades prior.
Ideally, Marwan's lack of connection and attachment to Damascus would allow him to move the Capital to Raqqa/Barbalisos. Navigable via the Euphrates to Kufa and Basra, allowing much more control. With Iraqi grain barges sent upstream allowing a huge population, being one of the only locations for a Syrian metropolis which can rival and surpass Kufa - the largest city in the Caliphate and it's intellectual hub. Though hostile to the Umayyads. https://www.alternatehistory.com/forum/threads/raqqa-ideal-caliphal-capital.544649/
Overall a metropolis, navigable to Iraq, should allow a much more centralised Caliphate of than OTL Umayyads, who ruled out of small desert castles...
Raqqa being on the Syrian-Jazira border would also give the Qaysi tribes much more integration in the state, especially since Marj Rahit never occured.
This means less of a total Syrian-Yamani monopoly, which should lessen the later Yamani-Qaysi rivalry.
Though since Iraq isn't demilitarised and is far less hostile without Karbala, the Syrian monopoly wouldn't really exist... Making Umayyad rule much more popular among the non-Syrian masses
After a short reign of 5 years, his son AbdulMalik would become Caliph.
Able to enact his brilliant reforms immediately, without need of 7 years of difficult campaigning against Umayyad claimants and Zubayrids.
His centralising policies having a much greater efficacy from the strategic capital of Raqqa
During his 20 year reign, expansion would be mainly focused on the unfathomable riches of India. As well as the anarchy next-door in the Byzantine empire.....
In 692, Sisebert, the metropolitan archbishop of Toledo, and many Visigothic nobles rebelled against King Egica, taking the capital of Toledo and briefly installing Suniefred as King, minting coins in his name.
The Muslims across the straits would use the chaos to invade the fractured Visigothic kingdom, in a campaign similar to 711. But with support from the Maghrebi navy.
Taking all Iberia by 696/7. Leaving the door open to Frankia, which was reunifying and becoming stronger under Pepin of Herstal. But it's still very fractured and vulnerable...
Overall this is a much more popular and stable Umayyad Caliphate. That might be able to last indefinitely....