With Hugh the Great's death in 956 Lothair, only fifteen, came under the guardianship of his maternal uncle Bruno, archbishop of Cologne, brother of East Francia's king Otto I.

The guardianship of Archbishop Bruno of Cologne lasted until 965 and oriented Lothair towards policy of submission towards East Francia, which was evolving into the German Holy Roman Empire.

[13] In August 978 Lothair mounted an expedition into Lorraine accompanied by Hugh Capet and upon their crossing the Meuse river took Aachen, but did not capture Otto II or Charles.

[15] Lothair was able to escape from the Imperial troops, but Charles was proclaimed King of the Franks[16] in Laon by Bishop Dietrich I of Metz, a relative of Emperor Otto I.

The Frankish royal army led by Lothair pursued and defeated them while crossing the river Aisne[9] and being able to recover Laon, forcing Otto II to flee and take refuge in Aachen with Charles, the puppet-King he wanted to impose on West Francia.

[17] Thus, written in 1015, the Chronicles of Sens gives an epic description: there Lothair was exalted as a warrior-king who pursued the German Emperor to the heart of Lorraine, destroying on the banks of the Argonne a great multitude of enemies, then returned to the Kingdom of the Franks covered with glory.

Contemporary documents speak of the event with the same triumphal accents: written after the retirement of Otto II, a diploma of the Abbey of Marmoutier near Tours dated during the reign "of the great King Lothair, in his twenty-sixth year (of rule; although apparently wrong), in which he attacked the Saxon and forced the Emperor to escape ".

On 8 June 979 Prince Louis was crowned as associated ruler or Junior King (iunior rex)[f][19] – although he did not actually assume power until Lothair's death in 986[20][21] – being the second time of this new practice in the kingdom of West Francia, which later was adapted by the Capetians.

[9] As a part of the agreement, Lothair renounced his rights to Lotharingia, allowing Otto II to turn his military attention to the Byzantine Italy, which he wanted to conquer.

The Peace of Margut led the Frankish kingdom to be included in the Ottonian orbit, and consequently weakened the influence of the Robertians within the royal government in favor of the Lotharingian nobility.

The project of Lothair was ambitious: the restoration of the royal Carolingian presence in the semi-independent south of West Francia, and – according to Richerus – to gain support of southern nobles in his fight against the Robertians.

The wedding between Adelaide-Blanche and Prince Louis took place in 982 at Vieille-Brioude, Haute-Loire, and they both were immediately crowned King and Queen of Aquitaine by Adelaide's brother Bishop Guy of le Puy.

However, the existence of the marriage, despite being recorded by relative contemporary and later sources (Richerus, Rodulfus Glaber, the Chronicon Andegavensi and the Chronicle of Saint-Maxence, among others), was recently challenged by historian Carlrichard Brülh.

Henry II, Duke of Bavaria, as the nearest Ottonian relative, obtained the regency of the kingdom without much opposition and abducted Otto III in hopes of being proclaimed king himself.

Nevertheless, the quick failure of Henry II's plans foiled the project: by mid-984 Empress Theophanu and Archbishop Willingis of Mainz were able to rescue Otto III and to regain control over Empire.

[25] The subsequent peace concluded at Worms between Henry II and Theophanu reaffirmed the end of the Carolingian pretensions over Lotharingia and the triumph of the House of Ardennes, which strengthened its control there.

Lothair refused to renounce the land which he considered rightfully belonging to him: he decided to make an alliance with Henry II, and on 1 February 985 their combined forces arrived to the banks of the Rhine in Brissach.

He also forced him to write to the Archbishops of Egbert of Trier, Willigis of Mainz and Ebergar of Cologne, stating that he, Lothair, was the true and only heir of the Carolingian Empire.

[25] When the King ordered the destruction of the fortifications that surrounded the monastery of Saint-Paul in Verdun, Adalberon refused on the grounds that his hungry soldiers no longer were able to keep the city.

In early 986 Lothair intended to attack Cambrai, an imperial city but one dependent on the Archbishoprics of Reims and Liège;[32] he thought that he could convince Bishop Rothard to surrender the city in exchange for his appointment as Archbishop of Rheims (following the deposition of Adalberon) and Prince-Bishop of Liège (whose Prince-Bishop Notger finally escaped to Ottonian territory[33]); but the King suddenly died at Laon on 2 March 986.