The Danish intervention began when Christian IV (1577–1648) the King of Denmark-Norway, himself a Lutheran, helped the German Protestants by leading an army against the Holy Roman Empire, fearing that Denmark's sovereignty as a Protestant nation was being threatened. The period began in 1625 and lasted till 1629. Christian IV had profited greatly from his policies in northern Germany (Hamburg had been forced to accept Danish sovereignty in 1621, and in 1623 the Danish heir apparent was made Administrator of the Prince-Bishopric of Verden. In 1635 he became Administrator of the Prince-Archbishopric of Bremen too.) As an administrator, Christian IV had done remarkably well, obtaining for his kingdom a level of stability and wealth that was virtually unmatched elsewhere in Europe, paid for by the Øresund toll and extensive war reparations from Sweden. It also helped that the French regent Cardinal Richelieu was willing to pay for a Danish incursion into Germany. Christian IV invaded at the head of a mercenary army of 20,000 men, but the Danish forces were severely beaten, and Christian IV had to sign an ignominious defeat, the first in a series of military setbacks to weaken his kingdom.