[Chancellor Palpatine]

Justinian spent amazing amounts of money in the late 6th century trying to bring Italy, Spain and North Africa back into the Empire, while pacifying the border with Persia in the East. And this was all while he was building a splendid capital, including the huge Hagia Sophia, which stands today.

What he couldn't foresee in all his adventures was the beginning of Islam several decades after his death, which came on like a supernova, and which no one was prepared to contain. I'm not sure that it could have been withstood past the fallback lines in Asia Minor due to terrain, logistical and communications issues, but I'd like to think that Syria, Lebanon, Israel and egypt could have held longer and wound up differently (those were predominantly ethnically Greek with some local mixing at that point in time). Anyway, considering how long the line held in Asia Minor, they didn't do that badly.

[Frumious Bandersnatch]

The thing is, if the internal rot hadn't overtaken it first, it is very doubtful that Byzantium would have fallen. At the time of Justinian, Islam would have been crushed rather decisively. However, the byzantine politics started with Justinian and seriously impaired Constantinople's ability to defend herself shortly after Justinian's death.

[Seydlitz]

There was nothing that made the rise of Islam inevitable. Rather, it hit the Byzantine and Persian empires with exquisite timing. Much of the early 7th century was dominated by a brutal and protracted war between the Byzantine and Persian empires. The Byzantines, led by the great soldier-emperor Heraclius, ultimately triumphed. However, both empires were exhausted by the struggle. They never fully recovered by the time the Islamic armies from Arabia hit them.

Moreover, both empires were exacting a very heavy tax on their citizens. The Muslims promised a substantial reduction in taxes to all who converted; non-believers had to pay a special poll-tax. Once this tax-exempt status was removed, the spread of Islam came to a dead halt. Charles Martel's admirers like to claim that he stopped the Arabs, but his victory was only won against a largish raiding party, not a real Arab army of conquest. It was the end of Muslim tax breaks that ultimately defined Islam's borders.