This paper investigates the Great War experience and social identities of the young Catholic priest from Austria-Hungary, Dr. Jozef Tiso. Born in 1887, Tiso became President of the Nazi-Germany-allied Slovakia in 1939-1945, and was executed by the Czechoslovak justice in 1947. This study mainly uses (and compares) Tiso’s diary, written during his service in the Habsburg army at the Austro-Russian front in 1914, and his 1946 affidavit to the Czechoslovak investigators. Exploring his manyfolder identity, we challenge the two dominant views that, before 1918, Tiso felt himself either a Slovak nationalist or a Magyar nationalist. We conclude that the social identity of Jozef Tiso in WWI consisted of two main categories: Catholic priest and Austro-Hungarian patriot. These categories coexisted and intertwined with those of Slovak sympathizer and Hungarian citizen.