The central theme in his work is the balance between nature and technology – he was called "the Green Poet" in Norwegian literature.
[3] Jacobsen's Jord og jern, written in free verse, introduced the urban world, racing cars, airplanes, and electrical turbines.
He did not share the Futurists' euphoria over modern inventions, the beauty of "a roaring motorcar, which runs like a machine-gun," but saw the relationship between machines and human civilization as more complex.
Jacobsen's diverse literary and other artistic influences included the Poetic Edda, Karel Čapek's play R.U.R., and Carl Sandburg's poetry.
He had joined a socialist intellectual group, Clarté, and in Åsnes he became a member of the Labor Party Leadership for Hedmark County.
Jacobsen rejected Marinetti's manifesto, "We wish to glory war...", but predicted the ominous emergence of the gas masks and machine guns.
During the Occupation of Norway by Nazi Germany (1940–1945), Jacobsen signed and published in Kongsvinger Arbeiderblad editorials that supported the German occupiers.
After the liberation of Norway at the end of World War II, Jacobsen was convicted of treason and sentenced to three and a half years at hard labor.
After his wife's death, in his last book, Nattåpent (1985), Jacobsen published tender and mournful poems about their life together: "Whoever loves for years / hasn't lived in vain."