Complex sentence: Reilly was arrested and interned for the duration of the war in prisoner of war camps that he developed his virtuosity on the harmonica, basing his ideas of phrasing and interpretation on the playing of Jascha Heifetz.
Split into simpler sentences: Reilly was arrested and interned for the duration of the war in prisoner of war camps. However it was there that he developed his virtuosity on the harmonica, basing his ideas of phrasing and interpretation on the playing of Jascha Heifetz.