When he became King he was forced to sign away most of his power to the nobles who controlled the wealth of the country. However, when Fredrik III declared war against Sweden, the nobles refused to support him. The city of Copenhagen was besieged but heroically withstood the onslaught, thanks to the people who rallied to the King. However, Denmark lost even more territory. The country was bankrupt and the land had been laid waste by the Swedish forces. In 1660 Fredrik III assumed absolute power for himself and his heirs, accomplished with the support of the townspeople and peasants, although this resulted in only a more centralized bureaucracy. With his councillor, Peder Schumacher, he then drew up the Lex Regia or Royal Law which created an absolute monarchy. 
 

Source: Leo van de Pas