When Maximilian realized that a few days at the most would decide his fate, he commissioned Basch, Lieutenant Ernst Pitner, and Major Becker to keep daily records of all that happened. When the emperor and his entourage were betrayed to Benito Juárez by Colonel Miguel Lopez on 14 May 1867, Basch rushed to saddle his horse, but was at once overpowered by the Mexicans. Basch lost most of his memoranda, saving only cursory notes. After the execution of Maximilian, he returned to Austria with the Emperor's body, arriving on 16 January 1868 on the Novara.
Basch's best-known work is Erinnerungen aus Mexico (1868), written at the request of Maximilian. In addition, he has written for technical journals a number of articles on the histology of the duodenum, the anatomy of the urinary bladder, and the physiological effects of nicotine.