He also governed under the name Frederick II as diocesan administrator (colloquially referred to as prince-bishop) of the Prince-Bishopric of Verden (1623–29 and again 1634–44), and the Prince-Archbishopric of Bremen (1635–45).

According to Cathal Nolan, when Frederick III became king in 1648, he was excluded from the talks leading to the Peace of Westphalia and had to watch as Bremen was lost to rivaling Sweden, also freed from paying Oresund Tolls.

[4] The death of his elder brother Christian in June 1647 opened the possibility for Frederick to be elected heir apparent to the Danish-Norwegian throne.

However, due to misgivings about the rule of Christian IV, as well as Frederick's previous confrontational administrations in Bremen and Verden and his quarrels with Anders Bille, he was only elected after he had signed a Haandfæstning charter.

He felt that temperament and policy would combine to make Charles an aggressive warrior-king: the only uncertainty was in which direction he would turn his arms first.

Charles's invasion of Poland in July 1655 came as a distinct relief to Frederick, even though the Polish War was full of latent peril to Denmark.

In the beginning of May, the still pending negotiations with that power were broken off, and on 1 June Frederick signed the manifesto justifying a war, which was never formally declared.

Yielding to the persuasions of the English and French ministers, Charles finally agreed to be content with mutilating, instead of annihilating, the Danish monarchy.

Frederick expressed the desire to make the personal acquaintance of his conqueror and Charles X consented to be his guest for three days, 3 to 5 March, at Frederiksborg Palace.

Splendid banquets lasting far into the night and intimate conversations between princes who had only just emerged from a mortal struggle seemed to point to nothing but peace and friendship in the future.

[4] During this war, Frederick attained great popularity in the general public, as he rebuked the advice of his counsellors to flee Copenhagen with the memorable words "I will die in my nest" and actively led the defense of the city.

But the government and the people displayed a memorable and exemplary energy under the constant supervision of the king and queen and mayor Nansen.

[4] So strong was the city by this time that Charles X, abandoning his original intention of carrying the place by assault, began a regular siege.

The traditional loyalty of the Danish middle classes was transformed into enthusiasm for the king personally, and for a brief period Frederick found himself the most popular man in his kingdom.

[3] During the last ten years of his reign, the king again took a relative obscure position while the new monarchy was built up and the country's people tried to recover after the wars.

New men came into government, which was marked by a rivalry between the ministers and councillors like Hannibal Sehested and the king's favourite Kristoffer Gabel.

[5] During the early and mid 1660s there was an influential circle around Sehested, Frederik Ahlefeldt, Peter Bülche, Hans Svane, Jacob Petersen and Theodor Lente, who became increasingly opposed to Gabel.

In 1665, Frederick had an opportunity to repay the Netherlands for their support, by protecting the Return Fleet from the Dutch East Indies from the English navy.

But before the Danish-Norwegian fleet or word of the deal reached Bergen, the English attacked, and were defeated in the Battle of Vågen by the Dutch, supported by the fortress.