[1] After his studies his cousin promoted Valdemar's provision for the See in Sleswick (Danish: Slesvig, German: Schleswig) in 1179, although still too young to be consecrated bishop as successor of the late Frederick I.
[2] Bishop Valdemar learned about the abbot, monks, and nuns at the Benedictine Abbey of St. Michael's in the town of Schleswig, who had fallen into immoral behavior and had earned a reputation for drunkenness.
[4] In 1190 Emperor Henry VI revoked Hartwig's princely regalia empowering his rule in the Prince-Archbishopric of Bremen, for his partisanship with the House of Welf.
In the same year the Bremian Chapter didn't wait any longer for a papal dismissal of Hartwig and unauthorisedly elected Valdemar as its new Prince-Archbishop,[5] encouraged by Henry VI.
Moreover, the bishop conspired with Adolphus III of Holstein, perhaps also with his brothers-in-law Jaromar I of Rugia and Casimir II of Pomerania, and with Canute I of Sweden and Sverre of Norway.
The following year he organised – supported by the Hohenstaufens – a fleet of 35 ships manned with Norwegian or Swedish mercenaries and harried the coasts of Denmark with an eye to overthrowing King Canute VI, claiming the Danish throne for himself, while Adolphus III crossed the Eider invading the duchy of Schleswig.
[7] Despite the interventions by Pope Celestine III Bishop Valdemar stayed in captivity in Nordborg (1193–1198) and then in the tower at Søborg Castle on Zealand until 1206.
[8] When in 1207 Hartwig of Uthlede died, a majority of Bremian Capitulars – overlooking the votes of the absent constitutionally provided three representatives of the Hamburg Concathedral chapter[11] – again elected Valdemar.
[6][12] A minority, led by Bremen's cathedral capitular provost Burchard of Stumpenhusen, who had opposed this election, fled to Hamburg, then under Danish occupation.
[9][11] Valdemar, still asserting himself as Prince-Archbishop, could not hinder Iso of Wölpe, Prince-Bishop of Verden, to capture the Bremian castle in Ottersberg.
The fled capitulars and King Valdemar II of Denmark then gained the Hamburg chapter to elect Burchard as anti-archbishop in early 1208.
[9] Prince-Archbishop Valdemar confederated with the free peasants of Stedingen, a region within the Prince-Archbishopric whose inhabitants rejected to be subjected as serfs, sparing them from socage.
[9] After Philip's assassination in June 1208, Prince-Archbishop Valdemar as well as the burghers and the city of Bremen joined the party of the former rival king Otto IV, whom Innocent III crowned Emperor in 1209.
In November 1210 Innocent III fell out with Otto IV, since the emperor claimed papal territory as imperial fiefs and demanded King Frederick Roger of Sicily to render the newly crowned Otto IV homage as vassal for the Duchy of Apulia and Calabria, two imperial fiefs Fredrick Roger held in personal union.
[15] Nevertheless, in the same year Henry V, his younger brother Otto IV, Margrave Albert II of Brandenburg, and Prince-Archbishop Valdemar and their troops, among them mercenaries from Stedingen, conquered Hamburg.
In 1216 the mercenaries of Stedingen swung over to the party of Gerhard I,[17] who promised to respect their freedom, and attacked the city of Bremen, loyal to Valdemar.
Gerhard's troops approached the fortress of Vörde disguised as sick, lining up for a treatment by the faith healer and farmer Otbert.
[12] After Valdemar had recovered, however, he had to do penance and went to Rome in 1220, where Pope Honorius III forgave him, lifted the anathema again, and reaccepted him in the Church's bosom, but forbade him to officiate as priest and sent him to the Cîteaux Abbey.
[20] Valdemar escaped the Loccum Abbey and gathered a crowd of supporters and invaded Danish-occupied Holstein in 1224, but was repelled by the Danish military commander Albert II, Count of Weimar-Orlamünde.
[20] Valdemar Knudsen did not recover, he was overlooked as participant in the Battle of Bornhöved (1227), where the victorious Low Saxon alliance of princes stopped the then Danish expansionism.