Complex sentence: He was shot in 1915, and spent the rest of the war as a prisoner of the Russians; while held in a Russian camp at Nikolsk-Ussuriysk in Siberia, Helly wrote important contributions on functional analysis.
Split into simpler sentences: He was shot in 1915, and spent the rest of the war as a prisoner of the Russians. While held in another camp at Nikolsk-Ussuriysk, also in Siberia, Helly wrote important contributions on functional analysis.