In 1905–1907, Rei participated in the Russian revolution of 1905. In 1906, he edited the underground paper "Sotsiaaldemokraat" ("Social Democrat") in Tallinn. Between 1912 and 1913, he was in compulsory army service. In 1913–1914 he worked as a lawyer in Viljandi. In 1914–1917 Rei was an artillery officer in World War I at the Fort of St. Petersburg. In 1917 and 1918 he was in the Estonian army units (formed after the Second Russian Revolution in March 1917 by the initiative of Estonian politicians, who achieved the permission to concentrate the Estonian soldiers). He was the Assistant Chairman of the Supreme Committee of the Estonian military. In 1917 and 1919, he was the Editor-in-Chief of the paper "Sotsiaaldemokraat". Between 1927 and 1928 he was the editor of "Rahva Sõna" ("Word of the People"). Rei was one of the leaders of the moderate faction of Estonia's social democratic movement. Up to 1936 he also worked as a lawyer and defended Ado Birk in 1927.