Complex sentence: After 1939, the North West Frontier quietened down, and indeed remained reasonably peaceful, albeit apart from the occasional raid on a village or attack on a garrison, until the end of British rule in 1947.
Split into simpler sentences: After 1939, the North West Frontier quieted down, and remained reasonably peaceful. Apart from the occasional raid on a village or attack on a garrison, things would remain this way until the end of British rule in 1947.