Stahel was born in Szeged, Kingdom of Hungary, 200 km southeast of Budapest. After schooling in Szeged and Budapest, he entered the Austrian Army, rising to the rank of lieutenant. Stahel joined the movement for Hungarian independence led by Lajos Kossuth during the Revolution of 1848. He served on the staffs of Gen. Artúr Görgey and Gen. Richard Debaufre Guyon. When the Hungarian uprising against the Habsburg dynasty was put down in 1849, he fled to Prussia and then to England before migrating to the United States in 1859. Until the outbreak of the American Civil War, he worked for "Deutsche illustrirte Familienblätter", a German-language newspaper in New York City.