The death of the emperor in 840 was the signal for the outbreak of war between his sons. Charles allied himself with his brother Louis the German to resist the pretensions of the emperor Lothar, and the two allies conquered him at Fontenoy-en-Puisaye on 25 June 841. In the following year, the two brothers confirmed their alliance by the celebrated oaths of Strassburg. The war was brought to an end by the treaty of Verdun in August 843. The settlement gave Charles the Bald the kingdom of the western Franks, which practically corresponded with what is now France, as far as the Meuse, the Saône and the Rhone, with the addition of the Spanish March as far as the Ebro.
The first years of Charles' reign, up to the death of Lothar I in 855, were comparatively peaceful, and during them was continued the system of "confraternal government" of the sons of Louis the Pious, who had various meetings with one another, at Coblenz (848), at Meersen (851), and at Attigny (854). In 858, Louis the German, summoned by disaffected nobles to oust Charles, invaded the western Frankish kingdom. Charles fled to Burgundy, and was only saved by the help of the bishops and by the fidelity of the Welfs, who were related to his mother, Judith. In 860 he in his turn tried to seize the kingdom of his nephew, Charles of Provence, but met with a repulse. On the death of his nephew Lothar II in 869, Charles tried to seize Lothar's dominions, but by the treaty of Mersen (870) was compelled to share them with Louis the German.
Besides these family disputes, Charles had to struggle against the incessant rebellions in Aquitaine and against the Bretons. Led by their chiefs Nomenoë and Erispoë, who inflicted on the king the defeats of Ballon (845) and Juvardeil (851), the Bretons were somewhat successful. Charles also fought against the Normans, who devastated the country in the north of Gaul, the valleys of the Seine and Loire, and even up to the borders of Aquitaine. Charles was several times compelled to purchase their retreat at a heavy price. Charles led various expeditions against the invaders, and tried to put a barrier in their way by having fortified bridges built over all the rivers.
In 875, after the death of the emperor Louis II, Charles the Bald, supported by Pope John VIII, descended into Italy, receiving the royal crown at Pavia and the imperial crown at Rome (29th December). Louis the German, who was also a candidate for the succession of Louis II, revenged himself for Charles's success by invading and devastating his dominions. Charles was recalled to Francia, and after the death of Louis the German (28th August 876), in his turn made an attempt to seize his kingdom, but at Andernach met with a shameful defeat (8th October 876). In the meantime, John VIII, who was menaced by the Saracens, continued to urge Charles to come to Italy. After having taken at Quierzy the necessary measures for safeguarding the government of his dominions in his absence, Charles again crossed the Alps, but this expedition had been received with small enthusiasm by the nobles, and even by Boso, Charles's brother-in-law, who had been entrusted by him with the government of Lombardy, and they refused to come with their men to join the imperial army. At the same time Carloman, son of Louis the German, entered northern Italy. Charles, ill and in great distress, started on his way back to Gaul, and died while crossing the pass of the Mont Cenis on the 5th or 6th of October 877.
Charles was succeeded by his son, Louis, the child of Ermentrude, daughter of a count of Orleans, whom he had married in 842, and who had died in 869. In 870 Charles had married Richilde, who was descended from a noble family of Lorraine, but none of the children whom he had by her played a part of any importance. Charles seems to have been a prince of education and letters, a friend of the church, and conscious of the support he could find in the episcopate against his unruly nobles, for he chose his councillors for preference from among the higher clergy, as in the case of Guenelon of Sens, who betrayed him, or of Hincmar of Reims.