Complex sentence: The Balts were however more controversial since the Soviet authorities viewed them as Soviet citizens (the Soviet Union had annexed the independent Baltic states in 1940) and therefore regarded the Balts as traitors, and the internees feared death sentences.
Split into simpler sentences: The Balts were however a more controversial case, since they were viewed by the Soviet authorities as Soviet citizens (the Soviet Union had annexed the independent Baltic states in 1940). The Soviets therefore regarded the Balts as traitors, and they feared death sentences.