In 1919, Coler wrote a book called "Suomalaisten jääkärien parissa: muistelmia yhteistyön ajoilta 1915–1918" (With the Finnish Jägers 1915–1918), a curriculum vitae of his time in Finland during the First World War.

Now aimless like many other post-war junior officers, Coler had to take on more temporary civilian work but wanted to make his military training, education, and experience work for him, which landed him a job as the chief of a Finnish Schutzkorps, but he also made money writing romanticized war literature for a local magazine.

When the Nazi Party reintroduced conscription, Coler returned to Germany and reenlisted in the Wehrmacht, which gave him command of a regiment in the invasion of Poland in 1939.

[6] After the war, former subordinates of Coler testified in 1953 before the Düsseldorf district court that their commander was arrogant, strict, and unapproachable, was proud of his noble Prussian heritage, referred to himself in the third person, and demanded his orders be followed to the letter.

Former members of Feldkommandantur 810 also testified that Coler was a supporter of the Nazi Party and their policies, and that he boastfully alluded to personally shooting Poles and Jews during Fall Weiss.