[1] In 1640 and 1641 he was sent to Wismar to negotiate a treaty with the Swedish chancellor, Axel Oxenstierna, and, if possible, to bring about the marriage of Christian's son Frederick and Gustavus Adolphus's daughter Christina.

He made it his main objective to develop Norway's material resources, reorganize the army, fortifications and fiscal system; and he aimed at giving the country a more independent position in the union with Denmark.

He invaded Sweden four times; successfully defended Norway from attack; and, though without any particular military talent, won an engagement at Nysaker in 1644.

[1] After the war he renewed his reforming efforts, and during the years 1646–1647 strove to withdraw his governorship from the benumbing influence of the central administration at Copenhagen, and succeeded with the help of Christian IV in creating a separate defensive fleet for Norway and giving her partial control of her own finances.

The quarrel became acute when Sehested's semi-independent administration of the finances of Norway infringed upon Ulfeldt's functions as lord treasurer of the whole realm.

In November 1647, Ulfeldt carried his point, and a decree was issued that henceforth the Norwegian leaders should send their rents and taxes direct to Copenhagen.

Unable to meet the charges brought against him, he compromised matters by resigning his governorship and his senatorship, and surrendering all his private property in Norway to the crown.

When, at last, it seemed probable that the war would not result in the annihilation of Denmark-Norway, Sehested strained every nerve to secure his own future by working in the interests of his native land while still residing in Sweden.

[3] Politically, he played a major role in those years within the queen Sophie Amalie's party, which included men like Jacob Petersen.

[citation needed] As a diplomat, he in some ways anticipated the views of Peter, count Griffenfeldt, supporting the policy of friendship with Sweden and a French alliance.