The Western Empire did indeed hold out long enough to suffer the wrath of the Huns, but just barely so. The Empire had devolved into a collection of loosely aligned barbarian kingdoms, which were politically and culturally independent and could hardly be called upon for mutual defense unless their own interests were immediately under threat.
Rome had abandoned Britain and the lands of North Africa that were the conquests of a young Republic during the Punic Wars, including Carthage, were now seized by Vandal pirates. Rome, had been ransacked by the Goths and had not been the Empire's capital for centuries, inconveniently located midway down the Italian peninsula and considered too far away for political or military expediency, and was given over to Milan, which was more strategically placed in the north with quicker access to the Alps and the provinces of Gaul and Germania. At this point in history, however, even Milan had been abandoned in favour of Ravenna, considered more easily defended in the marshlands boarding the Adriatic, and the imperial court ruled over the lands, nominally, from this hideout in the swamps.
Although the Huns had already plagued the Empire indirectly for some time, displacing other tribes that caused chaos and instability in the European provinces, a direct confrontation was yet years in the making. The Huns had a good public-relations machine in the reputation that preceded them that was talked up by fleeing refugees, and when Rome, nervous over this looming threat, offered to pay a tribute of a sizable amount of gold to the Huns in exchange for peace, they gladly accepted. Despite their attested prowess in battle and their later depictions, the Huns under the leadership of Attila were not mindless brutes intent on destroying civilisation but were rather content to keep to the periphery and collect their annual allowance. Like the Gothic, Vandals, Alans and the Franks, whom were soon to rebrand Gaul as France after their tribe, many Huns rose to prominence in the Roman ranks and fought for the Empire as mercenaries.
Meanwhile, back in the swamp, the sister of the Western Emperor Valentinian III, Justa Grata Honoria, was being strongly coerced into giving up her bon-vivant lifestyle and settle down and marry in a manner more becoming to an Augusta. Faced with the prospect of being wed to a perfectly boring, unambitious senator and remaining in Ravenna, Honoria penned a letter to Attila the Hun asking him to save her from this fate and enclosed a ring—at least, that is how the story goes. Whether it really happened, whether it was meant as a proposal or whether the ring was just a token of authenticity, is debatable and though some belief that this was Attila's impetus to invade the West, the Huns first skirted Italy, despite pledges of half the Empire as dowry, and invaded Gaul and never overran Ravenna. As Honoria gets no further mention, it looks like she received her lot with a boring, domestic existence as punishment for her act of treason. Perhaps realising that Rome was weak and the obvious choice for expanding his tribe's holdings, Attila led his armies through Germania and crossed the Rhine into Gaul.
Conveniently, there was a crisis of succession happening at the time for the Salian Franks. The Merovingian king had passed away and Rome and the Huns championed the younger and elder sons, Childeric at the court in Orlรฉans (Aurelianum) and Chlodio having teamed up with the Huns on their march through Thurginen, respectively—the Huns again plying their P-R apparatus by forging alliances and sowing discord and confusion among the status quo. Repulsed by a coalition of fighters under the leadership of Roman general Flavius Aรซtius at Chรขlons on the stoop of Orlรฉans, the Huns retreated, bidding a destructive exit back east by way of the northern Italian plain, and Chlodio—usurper or rightful heir was killed in the battle. Maybe the lore behind this proxy-coup is a little like the pseudo-history of one spoiled Roman princess' overture to the Hunnic chieftain, but I think the outcome of this intrigue bares mentioning:
Childeric, the homebody, inherited the Merovingian throne and founded a dynasty that ruled the expanding, united Kingdom of the Franks that filled the power vacuum after Rome fell for three centuries until the papacy anointed the Carolingian branch, leading to the crowning of Charlemagne as Emperor of the Holy and Roman Empire of the Germans, depriving the Merovingians of the right to rule. I am not the scholar to investigate all the reasons and motivations and the far easier course of action is to rehash the ancient patriarchal conspiracies that have some popular currency and persuasion: the Merovingian line was descended directly from the offspring of Jesus and Mary Magdalene, and rightful heirs to Church and State—whereas the Carolingians and other royal houses were descendants of Jesus' marginalised brothers and sisters or just plain self-made aristocrats that could claim no divine lineage. The Church feared the legitimacy of the Merovingians and wanted to install a more pliable set as heads of state.