By 450, Roman authority over Gaul had been restored in much of the province, although control over all of the provinces beyond Italy was continuing to diminish. Armorica was only nominally part of the empire, and Germanic tribes occupying Roman territory had been forcibly settled and bound by treaty as Foederati under their own leaders. Northern Gaul between the Rhine north of Xanten and the Lys (Germania Inferior) had unofficially been abandoned to the Salian Franks. The Visigoths on the Garonne were growing restive, but still holding to their treaty. The Burgundians in Sapaudia were more submissive, but likewise awaiting an opening for revolt. The Alans on the Loire and in Valentinois were more loyal, having served the Romans since the defeat of Jovinus in 411 and the siege of Bazas in 414. The parts of Gaul still securely in Roman control were the Mediterranean coastline; a region including Aurelianum (present-day Orléans) along the Seine and the Loire as far north as Soissons and Arras; the middle and upper Rhine to Cologne; and downstream along the Rhône.
The historian Jordanes states that Attila was enticed by the Vandal king Genseric to wage war on the Visigoths. At the same time, Genseric would attempt to sow strife between the Visigoths and the Western Roman Empire. However, Jordanes' account of Gothic history is notoriously biased and unreliable, and much of it is omitted or garbled. Other contemporary writers offer different motivations: Justa Grata Honoria, the sister of the emperor Valentinian III, had been betrothed to the former consul Herculanus the year before. In 450, she sent the eunuch Hyacinthus to the Hunnic king asking for Attila's help in escaping her confinement, with her ring as proof of the letter's legitimacy. Allegedly Attila interpreted it as offering her hand in marriage, and he had claimed half of the empire as a dowry. He demanded Honoria to be delivered along with the dowry. Valentinian rejected these demands, and Attila used it as an excuse to launch a destructive campaign through Gaul. Hughes suggests that the reality of this interpretation should be that Honoria was using Attila's status as honorary Magister Militum for political leverage.
Another possible explanation is that in 449, the King of the Franks, Chlodio, died. Aetius had adopted the younger son of Chlodio to secure the Rhine Frontier, and the elder son had fled to the court of Attila. It's thought that Childeric I was a vassal of Attila, and the founders of the Merovingian dynasty, Childeric and Merovech, are the two claimants to the Frankish throne. In the somewhat garbled story of the Chronicle of Fredegar, Childeric was expelled by the Franks and allegedly exiled for eight years to Thuringia, which was a Hunnic vassal at the time. Kim concludes that the character of Wiomad represents the Huns who helped Childeric fight the Romans and engineered his return from exile, stating that the main objective of Attila at Chalons was conquest of the Franks and establishment of vassal states on the Rhine.
Attila crossed the Rhine early in 451 with his followers and a large number of allies, sacking Divodurum (now Metz) on April 7. Other cities attacked can be determined by the hagiographies written to commemorate their bishops: Nicasius of Rheims was slaughtered before the altar of his church in Reims; Servatius of Tongeren is alleged to have saved Tongeren with his prayers, as Genevieve is to have saved Paris. Lupus, bishop of Troyes, is also credited with saving his city by meeting Attila in person. Many other cities also claim to have been attacked in these accounts, although archaeological evidence shows no destruction layer dating to the timeframe of the invasion. The most likely explanation for Attila's widespread devastation of Gaul is that Attila's main column crossed the Rhine at Worms or Mainz and then marched to Trier, Metz, Reims, and finally Orleans, while sending a small detachment north into Frankish territory to plunder the countryside. This explanation would support the literary evidence claiming North Gaul was attacked, and the archaeological evidence showing major population centers were not sacked.
Attila's army had reached Aurelianum (modern Orleans, France) before June. According to Jordanes, the Alan king Sangiban, whose Foederati realm included Aurelianum, had promised to open the city gates. This siege is confirmed by the account of the Vita S. Aniani and in the later account of Gregory of Tours, although Sangiban's name does not appear in their accounts. However, the inhabitants of Aurelianum shut their gates against the advancing invaders, and Attila began to besiege the city, while he waited for Sangiban to deliver on his promise. There are two different accounts of the siege of Aurelianum, and Hughes suggests that combining them provides a better understanding of what actually happened. After four days of heavy rain, Attila began his final assault on June 14, which was broken due to the approach of the Roman coalition. Modern scholars tend to agree that the siege of Aurelianum was the high point of Attila's attack on the West, and the staunch Alan defence of the city was the real decisive factor in the war of 451. Contrary to Jordanes, the Alans were never planning to defect as they were the loyal backbone of the Roman defence in Gaul.