Let's say that, for whatever reason, Eastern Roman Emperor Justinian I decides against invading Italy or going further West. Let's say he still invades the territories held by the Vandals in Tunisia, and that he's able to cling on to power. But that's as far West as he goes. Instead, he focuses on consolidating his Eastern holdings, with maybe a couple wars in the Middle East and the Balkans.

What do you think are the short term and long-term effects of this decision? Will the Emperor be able to hold onto power? Would the Eastern Roman Empire, less spent from costly wars trying to reconquer the West, be able to defend better against future Arab invasions? How do Ostrogothic Italy and Visigothic Iberia develop?
 
The conquests of Justinian did place a strain on the empire , both financially and in terms of manpower . But the Arab invasions were so successful because of the specific circumstances . The empire had gone through a period of massive instability . Several emperors were overthrown in short succession and that gave an opening to the Persians . What followed was nearly 3 decades of almost constant war against the Persians that left both empires almost bled dry , thus creating the perfect opening for the Arab conquests .
The Justinian Restoration didn't much affect Visigothic Spain all that much . In order for the fate of the penninsula to change , something different would need to happen in the Arab-Byzantine wars . If nothing changes , the Arabs still show up on schedule and history repeats itself .
Ostrogothic Italy is a far more interesting situation . Without Belisarius ruining the Ostrogothic state , they would have the potential of even holding it together for some time . That would make them less likely to be invaded by the Lombards , so north Italy could look completely different ITTL . Also , a large and robust Ostrogothic state in Italy severely limits the influence the Franks had over the Papacy . If the Pope doesn't need Charlemagne , its less likely to name him Emperor of the Romans . There is even the potential that the Italian state never fractures , leading to the awakening of an Italian national identity ( probably considerably different to the one they have now ) far earlier and Italy being one of the first nation states to emerge ( so Italy being more like France and less like the HRE ) .
 

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On the other hand a stronger ERE at the end of Justinian's rule thanks to no Italian campaign means that there might be enough ressources to more decisively and more quickly defeat the Persians, the two foes not being so equally well matched. This in turn changes the dynamic of the Arab conquests. About Italy, I could indeed see it go as nickmn says.
 
Let's say that, for whatever reason, Eastern Roman Emperor Justinian I decides against invading Italy or going further West. Let's say he still invades the territories held by the Vandals in Tunisia, and that he's able to cling on to power. But that's as far West as he goes. Instead, he focuses on consolidating his Eastern holdings, with maybe a couple wars in the Middle East and the Balkans.

What do you think are the short term and long-term effects of this decision? Will the Emperor be able to hold onto power? Would the Eastern Roman Empire, less spent from costly wars trying to reconquer the West, be able to defend better against future Arab invasions? How do Ostrogothic Italy and Visigothic Iberia develop?
Throw in Sicily and call it a day. That would give Constantinople the dominance of the Mediterranean that they needed to be the Superpower of the day. Becoming allies with Visa gothic Italy would go a long way to protect Roman interests in the West, and if that holds, they'd be in a better position to defend against the Persians. Of course, trying to predict how an alliance would hold up in 70 years is almost impossible. The plagues of the 6th Century didn't help ether.
 
As I understand it, part of why Justinian felt so free to wage war in the west was the 'perpetual peace' treaty with the Persians, which Khosrau I broke anyway. If Justinian believes he can secure a better peace in the east while Persia is still in some turmoil due to a recent succession, perhaps with one of the battles that the Persians won prior to the treaty OTL going the other way, then that might be a factor to pushing further east. One would question just how far east the ERE can push it's border though - the Mediterranean allows whoever is the major naval power in the region to project power anywhere in the surrounding areas, while holding onto Mesopotamia from the west has always been tricky due to the rivers flowing to the gulf of Persia. and the many mountains and deserts in the area.
 
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