Here are the notes from the Oak Publications German Folksongs book, apparently written by editor Arthur Kevess:

One of the most popular German folksongs, written during the Napoleonic wars. Words, 1809, by Ludwig Uhland, poet, historian, and professor of German literature at the University of Tübingen in his home town. Sixteen years later, the university's music director, Friedrich Silcher, dusted off a 3/4 time folk melody, "Ein schwarzbraunes Mädchen hat ein'n Feldjäger lieb" (which he considered to be of Swiss origin), changed it to a 4/4 marching tempo, and fitted Uhland's words to it. The song, included in his Folksongs, 1827, has been a favorite ever since.

During the Spanish Civil War (1936-39), "Ich hatt einen Kameraden" was widely sung by the anti-fascist volunteers of the Thälmann Battalion. Marching to or from the front, and especially when mourning their losses, they sang this song of the "good comrade."

Singer Ernst Busch conceived the idea of giving the song greater meaning. His new lyrics for "Hans Beimler" soon replaced the original ones on the volunteers' lips. Loyalist partisans here (in the U.S.??) sang the new song both in German as well as in a semi-singable unrhymed anonymous translation.

Hans Beimler had been a deputy in the Bavarian Diet, or parliament. Sent to Dachau in 1933, he was one of the few prisoners who ever escaped from the camp. He went to Spain as a leader fo the first contingent of International Brigade volunteers who helped save Madrid in November 1936. Chief political comissar, he was killed in action in December of that year.