After World War II, he found out about the lies and atrocities of the Nazi regime, primarily by following the Nuremberg trials and reading Eugen Kogon's book Der SS-Staat [de].

He earned a PhD in Tübingen with his thesis "Germany and Japan between the Powers, 1935–1940",[5] supervised by Hans Rothfels.

[6] He began his career as a journalist with the Rems-Zeitung [de], a local paper in Schwäbisch Gmünd.

His editorials for Die Zeit in the 1970s shaped the paper's social-liberal attitude, and supported the policy of détente with the Eastern bloc states (Entspannungspolitik).

[2][10] In 2016, American historian Alexander J. Motyl criticised Sommer for "closing his eyes to the mass murders of the Soviet regime", "disregard" for the Baltic states and Poland, and a "classically colonial" attitude toward Ukraine.

[9] According to Die Zeit, "he decisively shaped" the paper into a "cosmopolitan, liberal publication, [which] welcomes debate" with "his temperament, his energy, his shrewd judgment and his cheerfulness" ("mit seinem Temperament, seiner Tatkraft, seinem klugen Urteil und seiner Fröhlichkeit als weltoffenes, liberales, debattenfreudiges Blatt maßgeblich geprägt hat").