It seems a popular theory is the over extension of the Byz emperor by Justinian led to a weakening of the empire overall and led to a future disaster. IN my opinion after reading lots of Treadgold and Haldan etc i have a very different opinion of this and will explain why. It was really the massive Justinian plague that caused the disruption vs the invasion of the west. If the plague had not happened you would have had significant more resources to defend in the east or even expand in the east.
Justinian strategy :
1. Make peace in the East to reconquer the West
2. retake a poorly defended west, with clearly defensive able borders, which provided a positive cash flow
3. Use these resources to increase fortifications and military forces to better defend in the east
Notes
1. Vandals were determined to be weaker then in the past, with defensive able borders to the south
2. Chaos in Italy with the change of leadership
Time frame
1. 527 - 532 war in the east with some success and defeat --- with peace treaty in 533
2. 530 ::: Gelimer overthrows leader of the Vandals and is hostile to Byz
3. 533 to 534 -- Conquest of Vandals in NA --- the remnant of their army deported to fight for Byz in the East --- massive treasure captured and transported to Const. (MISTAKE 1. NOT LEAVING SUFFICIENT FUNDS TO SUPPORT OCCUPATION FORCES -- IF THEY HAD KEPT ENOUGH THE MUTINY OF THE FORCES WOULD NOT HAVE HAPPENED)
4. 535 - 540 --- Italy captured -- (IF JUSTINIAN HAD WAITED ON BUILDING THE HAGIA SOPHIA (532) AND UTILIZED THESE FUNDS TO ADD 10,000 - 15,000 MORE TROOPS TO BELISAURIS this would have made a massive difference) however Italy was taken at this time. So a strong and prosperous Italy with defensible borders (Alps) and allies to the west (Franks) and the East (Lombard's). The mop up of the Goths would have been relative easy if 542 had not happened.
5. 542 -- the Plague of Justinian -- (read Justinian flea) -- over the course of the next 10 years their are est that up to 1/3 of the population died. This disrupted the flow of taxes, soldiers died, and trade ground to a halt.
So you have a situation where the Persians attacked in 540, the Plague in 542, and the mop up of the Goths in Italy was delayed.
If you had stabilized Italy in the period of 535 - 540 (10,000 more troops vs building in Const) -- you would have had massive more resources to fight in the East. If no Plague you would have had about 45% more population and 35% more excess income to defend in the east.
I think an argument can be made that even with the Plague with out the conquests in the West the Byz would have fallen to the Persians.