In the summer of 1870, Mosengel met Friedrich Nietzsche and his sister, who were spending the season in the Maderanertal, a valley at the northern foot of the Saint-Gotthard Massif which has been compared to the Mount Parnassus. Mosengel and the Nietzsches were staying in the Hotel Alpenclub. The Franco-Prussian War was about to break out, and Nietzsche had been considering joining the Prussian medical corps; apparently Mosengel strengthened [his] resolve to do so. Besides Leopold Rau, Mosengel is likely the only painter Nietzsche knew personally.