Stamboliyski was a member of the Agrarian Union, an agrarian peasant movement which was not allied to the monarchy, and edited their newspaper. He opposed the country's participation in World War I and its support for the Central Powers. In a famous incident during 1914 Stamboliyski's patriotism was challenged when members of the Bulgarian parliament questioned whether he was Bulgarian or not, to which he shouted in response "At a moment, like the current, when our brothers South Slavs are threatened, I am neither a Bulgarian nor a Serb, I am a South Slav (Yugoslav)!". This statement relates to his belief in a Balkan Federation which would unite the region and supersede many of the national identities which existed at the time. He was court-martialed and sentenced to life in prison in 1915 due to his opposition to Bulgaria joining the Central Powers in WWI.