Cronau landed the job as their correspondent to the United States and sailed on the vessel "Oder" from Bremen, Germany reaching New York City on 17 January 1881.

In the fall of 1881 he journeyed to Fort Randall in Dakota Territory where he would meet and befriend Sitting Bull, then a prisoner of war at that post.

Cronau was sympathetic to the Indian's plight, causing him to remain nearly half a year in the territory documenting and illustrating the Sioux.

"[11] Cronau was aligned with the beliefs of Social Darwinism; he "argued that the key to progress was the annihilation of the "lower races," who stood in the way of advanced culture and civilization."

Some social Darwinists of his time believed that violent racial extermination "would result in moral progress for humanity.