How to make Justinians conquests last longer

Pushing the Roman gains in Spain to the Pyranese is doable, as are conquests in the Balkans for more defendable borders. That's about as good as Justinian could hope for. Natural, defendable borders, peace with the Persians, and a bright, capable heir - do this, and it's bright days ahead for the East Roman Empire.
OTL:
Achaemenid governance deported Greek dissendents to Bactria.
Julius Nepos' dominion ended in 480.
https://en.wikipedia.org/wiki/Category:Generals_of_Justinian_I
https://en.wikipedia.org/wiki/Category:Ministers_of_Justinian_I
535-536 extreme weather.
541-542 started Plague of Justinian
In 557, Avars contacted Constantinople.
https://en.wikipedia.org/wiki/Tomislav_of_Croatia
If Tomislav of Croatia in early 10th century can administrate south of the border against Magyar nomads -- horse people and whatnots. Tomislav to his administrative credits contacted Constantinople and helped managed Roman coastal cities for a profit. Numerical exaggeration offered Croatian military might of 100,000 infantrymen, 60,000 horsemen and a sizable fleet of 80 large ships and 100 smaller vessels.

ATL
I would encourage holding onto Illyricum (Croatia, Slovenia, Bosnia), instead of expedition to Spain. River Drava can be a natural border. How the governors of newly recovered Illyricum from Ostrogoths would be important.
Garrisons in Illyricum would make up to 20,000 infantrymen, 6,000 horsemen, fleet of 10 large ships and 30 smaller vessels.
Shall Constantinople want all Heruli, Gepid and Lombard foederati horse people knowing that Lombards would be the most capable of the three. If supporting Herules and Gepids against Lombards after the reconquest of the Peninsula was the method, Ostrogothic remnants and Lombards would join forces with the Avars when the latter group encountered the area in 560s. In other words, Constantinople needed a good manager who would be prefer politicking than direct conflict. Narses appeared to be of the politicking mold than Belisarius.
Given that the extreme weather in 530s would not favor direct military conflict in the long run, politicking means by Narses would become favorable.

Belisarius to his credit could be deployed near Armenia, where he campaigned against the Sassanids since 530s after his Dara's victory. To strength his army with cavalry, Belisarius would be recruited horse people in the Caucasian mountains where Avars would already be living north of it. Belisarius's management would have known the Avar's existence in 550s.

If Narses could conclude the conquest in the Italian Peninsula much earlier in 540s, a 20s to 30s years window for Constantinople to manage the economy, faith and military; 540s would be about 60 years after 480. Some population and their descendants would still remember Roman management in good favour over Ostrogothic: this could be a favorable factor for Narses' management.

So Narses in Ravenna, Belisarius in Armenia or Illyricum, Mundus in north of Drave managing Herules and Gepdis. All three holding on and waited for the final showdown against Lombards and Avars. The plague favored defensive measures over offensive against Avars and Lombards. Biological warfare could mean catapulting plague infected organics from the besieged Byzantium towns in Illyricum against the besieging Avars and Lombards nomads.

Another venue of revenue would be maintaining maritime trade thru the Red Sea to Indian subcontinent, Southeast Asia and East Asia bypassing the Persians and Arabian tribes. Keeping the bridgehead after the conquest of the Vandals could pay dividends to open a new trade route using Berbers to go across Eastern Sahara to the Christian Kingdom of Aksum when the eventful Islamic rise would hurt Roman economic interest in Egypt. By that time, a century of trade network of Romanized Berbers, Vandals and "other peoples" could be possible. Allowing me to stretching the politicking of Roman governance in Italian Peninsula and Constantinople, Ostrogoths, Vandals, Herules, Gepids, captured Lombards would all be deported to recaptured Vandal kingdom in North Africa. They would be living together with Berbers locals and acting as Roman caravan traders to Aksum thru the Sahara and via Egypt. If Orthodox church in Constantinople would be a wielding influence, all East Germanic deported tribe people (Gepids, Herules, Scrii, Vandals, Ostrogoths in North Africa would be using a developed script by priest from Constantinople, with reference to the works by Saint Cyril and Methodius for the Slavs. With a common script, residence in towns and caravan trade network, a common identity in Byzantium North Africa could surface in time when the Islamic conquest came. By the same ideas, Germanic peoples already localized in Italy and Illyricum would also be using that script, creating a common identity from faith in orthodox, residence in towns, and caravan trade network thru towns in the highlands of modern day Bosnia and Croatia or small vessel sailing downstream on Drava, to the Danube, to the Black Sea and to Constantinople.
 
If they just hadn’t tried to rush it and cheap out then his conquests would’ve gone much smoother. They should’ve stabilized Carthage so they didn’t have to deal with rebellions and given belisarius the necessary troops to take Italy without causing too much damage. The conquest of Spain and Gaul in the short term just isn’t feasible, they’re too far away to hold. In the long run though it might be doable. Once Africa and Italy are successfully integrated the Empire will have a much larger tax base and recruiting area. But to be honest Spain and Gaul aren’t worth it at this point. It’s itally, Illyria, and Carthage that you want. Just bully there rulers into sending tribute and mercenaries and call it a day.
 
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