Complex sentence: One of his songs, set to Lydia Koidula's poem "Mu isamaa on minu arm", became an unofficial national anthem during the years of Soviet occupation; ironically, he was also the composer of the Anthem of Estonian SSR used between 1945 and 1990.
Split into simpler sentences: One of his songs, set to Lydia Koidula's poem "Mu isamaa on minu arm", became an unofficial national anthem during the years of Soviet occupation. He was also the composer of the Anthem of Estonian SSR used between 1945 and 1990.