In 1808 he became secretary to the last Spanish governor, Francisco Antonio García Carrasco, and used his position to prepare the nationalist movement that began in 1809. After resigning his position as secretary, Rozas was mainly responsible for the resignation of the Spanish governor, and the formation of a national Junta on September 18, 1810 of which he was the real leader. After the death of the President and Vice President of the First Government Junta, he acted as Interim President.