From 90 BC (When Maius "reformed" the Roman army and made it a Mercenary Army) till after the Battle of Adrianople (A.D. 378), the Roman Army, while Mercenary was overwhelmingly Roman in makeup. In the Eastern Empire it appears to stay Roman (and that was a fact in why the Eastern Empire Survived till the Renaissance, The Eastern Capital Constantinople would fall to the Turks in 1453AD).
On the other hand the Western Empire seems to have embraced the Germans for its troops. It is only with this embrace that the Western Empire "Fell" (And this more after the Goths took Rome in 410 AD than before 410 AD). While the Roman Army seems to start to dissolved after 410 AD, parts of it survived to defeat Attila the Hun in the battle of Châlons in 451 AD. While the Roman Army after 451 AD was Overwhelmingly German to say that Roman Empire in the West fell in this period is a very inaccurate term. Roman Traditions and Roman Law survived till Charlemagne's Time (c800AD) and when you are looking at the "Native" Romans, while the "Invading" Germans retained their own law throughout the "Dark Ages" most of the "Roman" Subjects stayed under "Roman Law". Throughout the Middle Ages you had tension between these two systems that was not even resolved with the "restoration" of Roman law during the Renaissance.
The interesting fact was that part of the Roman Empire that retained "Romans" as the base for its army survived (i.e. The Eastern Empire), while the part of the Roman Empire that opt for Foreign Mercenaries fell within 50 years of that decision (i.e. the Western Empire, and I may say it may have fell within 20 years depending on how you look at the period 451-476 when Germans were running the Western Empire AND that the last real Roman Forces dissolved after the death of Aetius in 454 AD (For more in Aetius See
http://www.newadvent.org/cathen/01177b.htm Aetius is known as the man who Defeated Attila the Hun at Châlons in 451 AD).
Now from the time of Hannibal (c200BC), Rome had hired "Auxiliaries" from its subject people, but never left them get over 33% of their army (and mostly kept them less than 10%). One advantage of being in the Auxiliaries were you received Roman Citizenship for yourself and your family. By 212 AD so many people had become Roman Citizens either through serving in the Auxiliaries or otherwise became Roman Citizens that the Empire just declared everyone in the Empire a Roman Citizen. Thus by the time the Roman Empire in the West Fell (476AD) everyone within the Empire had been Citizens for over 260 years (With most people being Citizens for even longer periods). In many ways the Adoption of Christianity was an effort to bring one religion to the Empire for the older Pagan religions were to regional in scope to unite the Empire and thus only an Empire Wide Religion could do the job (and it appears that by 212 AD the people of what we call the Roman Empire did not use that term, preferring to called themselves Citizens of Romania).
Given the economic decay that Rome was in from the fall of the Republic onward (and the Roman's elites refusal to fix the problem for it meant giving up most of their wealth and power) it is surprising Rome was able to raise an army for as long as it did (In the East, The Eastern Empire dropped its Mercenary Army in response to the Persian invasion and than Arab Invasion of the Seventh Century, but Greece and right is now Turkey was the only area of the Roman Empire made up of small farmers instead of large estates). In the west when a choice came between arming their own peasants and hiring foreign mercenaries, the Romans opt for the foreign mercenaries for their main enemies was NOT the Germans but their own peasants.
When the Goths Marched on Rome in 410 AD, their army GREW as it marched as Roman Peasants joined it. This was characteristic of the Fall of Rome, Roman Peasants joining the Foreign "Invaders" instead of fighting them (The only exceptions seems to be if the invaders were Non-Christians, then opposition occurred, but if the invaders were Christians, such as the Goths, many peasants joined the "invaders"). Now the Romans in the West defeated every Invading German force (Except the Goths and Vandals) and once defeated put the defeated Germans in an area of a peasant revolt with orders to keep the peasants down. These Mercenaries were thus hired by Rome to keep their own people down. This worked for a few generations but as the Peasants and Germans intermingled the Germans increasing took the side of the Peasants over their Roman landlords. In many ways the removal of the last Western Emperor on 476 AD by his German military leader was more to remove Roman Interference with what the Germans were doing with the Romans Peasants than to kill the Empire (In fact the last Roman Emperor in the West was not even killed, he was just asked to resigned, which he did, and his General asked the Eastern Emperor in Constantinople to assume the Title of Western Emperor, as far as the Germans were concerned, the further the Emperor was the better).
I go into the above so that you have a better idea of ho the Western Empire Fell. The Germans did not invade it and destroyed the Empire. While the Germans (and the Huns and the Goths and other "Barbarian invaders") did enter the Empire, very few stayed in the Empire without Imperial Permission (And the two that stayed in the Empire without Permission, the Vandals and the Goths, both quickly developed popular support among the Roman Peasants they were ruling by giving the Peasants land, something most of them had not had since the time of Hannibal). While the Vandals were the first to give the Roman Peasants land (and the last to adopt Christianity) the rest of the German Invaders quickly followed. The Eastern Emperor objected to this and Ordered a Series of invasions (which Failed till the Rule of Justinian who managed to retake Italy, Tunisian and Southern Spain). Northern Italy fell to the Lombard's within a generation of Justinian's death. The Goths retook southern Spain about the same time (but lost it within another two generations to the Invading Arabs after they took Tunisia and Egypt).
The Arabs actually conquered that part of the Roman Empire that still had large estates (or had had large estates re-established as in Tunisia). The Arabs failed in areas where Rome had small farms (for example Right is now Turkey and Greece). While the Romans had re-established large estates in Southern Italy, the Arabs could not make any headway in Italy given the opposition from the Lombard's in Northern Italy (who had given their Roman Peasants land) and from the Eastern Empire out of Greece (and even here the Romans had a hard time holding onto Sicily and Naples and that is with the post-Arab invasion NON-mercenary Thematic army of the Eastern Empire NOT the Prue-Arab Conquest mercenary Army).
My point in the above was to show that Rome did not depend on Foreign mercenaries while it was strong, but only as it decayed. The Eastern Empire survived for it least depended on foreign mercenaries. Even the Eastern Empire had to quit relying on mercenaries once a real military threat appears (the Persians and than the Arabs) and go to a Universal Service Army (One whose costs include giving those troops a say in how the economy was run, i.e. gave them land when land was the major source of wealth).
In the Western Empire the selection of going with foreign mercenaries to put down your own peasants worked for a few generation but quickly lead to peasants getting the land as the foreign mercenaries realized that they did not need the Roman Elites just the peasants to be paid (and the Peasants will gladly pay their "superiors" what the "Superiors" want provided the peasants retained most of their results of their labor). It would take several centuries to complete, but with the Lombard Invasion of 570 AD you see Western Europe slowly getting rid of its Roman Elites. Many came back for Charlemagne (c800 AD), but by the Time of the Ottos and the Formation of the Holy Roman Empire in 900 AD, the old Roman Elites were gone (With the biggest "lost" between 800 and 900 AD as Charlemagne's Empire fell and the Otto's formed the Holy Roman Empire out of its broken Pieces.
For more on Hannibal:
http://www.barca.fsnet.co.uk/For more on Rome and Romania:
http://www.vic.com/~tscon/romanity/friesian/romania.htmhttp://www.friesian.com/romania.htmAn outline on the History of the Roman Empire (End to early in my mind, I believe the Empire in the West died with the Lombard Invasion of 570 AD NOT the resignation of the last Western Roman Emperor in 476, but gives you an outline of Roman History from the Republic to 476AD:
http://www.forumromanum.org/history/morey29.htmlFor maps on Rome see:
http://www.roman-empire.net/Here is a nice Paper on the Fall of the Roman Empire (nicer than my view that the problem was the Roman's elites failure to share the Wealth):
http://www.friesian.com/decdenc1.htmHere is a paper on one of the Sources for information at the time of the fall of the Western Empire:
http://etext.lib.virginia.edu/journals/EH/EH37/Goldberg.htmlSome Information on the Holy Roman Empire (800-1806AD)
http://www.heraldica.org/topics/national/hre.htmhttp://www.sciencedaily.com/encyclopedia/holy_roman_empire