The brothers joined in 523 to war against Godomar of Burgundy and his brother, Sigismund. (Clovis had defeated Godomar's father in 500, forcing Burgundy to pay tribute.) Godomar escaped the first encounter, but Chlodomer took Sigismund prisoner. Godomar rallied the Burgundians and regained his lost territory, but Chlodomer executed Sigismund. Fighting continued for a decade until 534 when Godomar was killed and Burgundy taken over.
Sadly for Chlodomer, he was killed in the final battle. Childebert and Clothar did not want his kingdom of Orléans to be divided among his three children, so they conspired to eliminate them. The eldest two were killed; the youngest escaped to a monastery. Childebert annexed Orléans and Chartres.
Future military campaigns gained him Geneva and Lyons. The king of the Ostrogoths ceded Provence to the Franks in 535; Childebert's share of the spoils were Arles and Marseilles.
He also invaded the Iberian Peninsula on behalf of his sister, Chrotilda. She had been married to King Amalaric of the Visigoths. (A purely political move: Amalaric's father Alaric II had been killed by Chrotilda's father, Clovis I. This marriage was supposed to cease national hostilities; it did nothing to assuage personal hostility.) She was Catholic; he pressured her to convert to the heretical Arianism. Gregory of Tours writes that he even beat her until she bled, and she sent a bloody towel to her brother.
Childebert attacked Amalaric, who fled but was assassinated. He brought his sister home, but she died along the way; he buried her in Paris next to their father. He also brought back the tunic of St. Vincent of Saragossa, patron of vintners, sailors, and brickmakers.
Childebert expanded his boundaries and built more religious structures than any of his brothers. He died on 13 December 558, leaving two daughters, who according to Salic Law could not inherit. His territory went to his younger brother Clothar I.
Here's a question: if Burgundy was already paying tribute to Gaul, was the war against Godomar necessary? Necessary, no; but motivated by a powerful force: a mother's wishes. I also left out a crucial and related detail regarding the disinheriting of Chlodomer's sons. I'll explain tomorrow.