From Wikipedia

The Real Attila The Hun

In 450 Attila proclaimed his intent to attack the powerful Visigoth kingdom of Toulouse, making an alliance with Emperor Valentinian III in order to do so. He had previously been on good terms with the Western Roman Empire and its de facto ruler Flavius Aëtius. Aëtius had spent a brief exile among the Huns in 433, and the troops Attila provided against the Goths and Bagaudae had helped earn him the largely honorary title of magister militum in the west. The gifts and diplomatic efforts of Geiseric, who opposed and feared the Visigoths, may also have influenced Attila's plans.

However Valentinian's sister was Honoria, who, in order to escape her forced betrothal to a Roman senator, had sent the Hunnish king a plea for help – and her engagement ring – in the spring of 450. Though Honoria may not have intended a proposal of marriage, Attila chose to interpret her message as such. He accepted, asking for half of the western Empire as dowry. When Valentinian discovered the plan, only the influence of his mother Galla Placidia convinced him to exile, rather than kill, Honoria. He also wrote to Attila strenuously denying the legitimacy of the supposed marriage proposal. Attila, not convinced, sent an emissary to Ravenna to proclaim that Honoria was innocent, that the proposal had been legitimate, and that he would come to claim what was rightfully his.

The general path of the Hun forces in the invasion of Gaul.

Attila interfered in a succession struggle after the death of a Frankish ruler. Attila supported the elder son, while Aëtius supported the younger. Attila gathered his vassals—Gepids, Ostrogoths, Rugians, Scirians, Heruls, Thuringians, Alans, Burgundians, among others and began his march west. In 451 he arrived in Belgica with an army exaggerated by Jordanes to half a million strong. J.B. Bury believes that Attila's intent, by the time he marched west, was to extend his kingdom – already the strongest on the continent – across Gaul to the Atlantic Ocean.

On April 7, he captured Metz. Other cities attacked can be determined by the hagiographic vitae written to commemorate their bishops: Nicasius was slaughtered before the altar of his church in Rheims; Servatus is alleged to have saved Tongeren with his prayers, as Saint Genevieve is to have saved Paris. Lupus, bishop of Troyes, is also credited with saving his city by meeting Attila in person.

Aëtius moved to oppose Attila, gathering troops from among the Franks, the Burgundians, and the Celts. A mission by Avitus, and Attila's continued westward advance, convinced the Visigoth king Theodoric I (Theodorid) to ally with the Romans. The combined armies reached Orleans ahead of Attila, thus checking and turning back the Hunnish advance. Aëtius gave chase and caught the Huns at a place usually assumed to be near Catalaunum (modern Châlons-en-Champagne). The two armies clashed in the Battle of Chalons, whose outcome is commonly considered to be a Pyrrhic victory for the Visigothic-Roman alliance. Theodoric was killed in the fighting and Aëtius failed to press his advantage, according to Edward Gibbon and Edward Creasy because he feared the consequences of an overwhelming Visigothic triumph as much as he did a defeat. From Aëtius' point of view, the best outcome was what occurred: Theodoric died, Attila was in retreat and disarray, and the Romans had the benefit of appearing victorious.