Where can I find evidence with sources and links) that the Western Roman Empire fell because they had to split their army and resources to protect against Parthia in one place and against Germanic tribes in another place? I'm having trouble with this because Parthia seems to have been defeated long before the empire began to deteriorate.
There are a couple of ideas that might help you sort this out. Indeed, the juxtaposition of Parthia with the fall of Rome is incongruous. Parthia itself fell in 224. Rome fell for good in about 476. What might help you is who replaced the Parthians in 224.
In 224 King Ardašir of Persia revolted from being a vassal of Rome and retook Ctesiphon. A massive fortune had already been taken from Ctesiphon to Rome by Septimius Severus in 198 leaving Parthia impoverished. When Ardašir attacked again, Parthia had no means of defense and fell. Parthia was replaced by the second Persian empire and was ruled by the Sasanians.
While the Western Roman Empire was defending its borders--with mercenary (hired) troops from many lands--to the north against the incursions of Germanic barbarians, the Sasanians were attacking in the east in Mesopotamia and later in Armenia.
Since the Sasanians formed the second Persian empire and are connected back in time to the Persians who defeated Parthia in 224, it might be said that the Parthians (or their eventual representatives) were attacking to the east while the barbarian Germanic tribes were attacking to the north.
Heritage History: Roman Persian Wars
I wanted to add: I HAVE to use this argument, so it's not really a choice... So even if you don't necessarily believe that this is a cause of the fall of the empire, please try to explain how you would prove this point (hypothetically).
Obviously half an army is weaker than the entire army, and using so many resources in two places would cause resources to deplete quickly, but I'm having a lot of trouble finding specific evidence and websites. The part about Parthia is especially causing me trouble because it seems to have been defeated way before the empire started to fall (Parthia = Persia...?). All I can find is lots of people saying a series of many (barbarian) invasions was a big cause in the fall, but I'm not trying to prove that barbarian invasions cause the empire to fall, I'm trying to prove it fell because it had to protect two different places and that stretched the empire thin... A lot of sites don't seem to mention Parthia or Persia much either.
Hope I make sense.