[2][3] In the 1760s he befriended Claude Louis de Saint-Germain, one of the most powerful men in Denmark-Norway at the time.

In 1766 he was promoted to lieutenant general, and on the initiative of Saint-Germain he was sent as Commander-in-chief of the Norwegian army, April 4 of the same year.

He found it indescribably boring to live in the country, and in a letter he wrote; ..this Land of the Devil, where there is not even a tree strong enough for a man to hang himself.

[4] During his time as Commander-in-chef, Rantzau was an important figure in development of several skiing sports.

When Stuensee became regent of Denmark and Norway, in December 1770, he gave Rantzau a central role in the new regime.