Complex sentence: During the Napoleonic Wars, the population plunged, it did not return to the 1,000s level until the mid 1810s, the population remained stable until the beginning of World War I, its highest mark that the census ever recorded was 1,159 in 1831.
Split into simpler sentences: During the Napoleonic Wars, the population plunged and did not return to the 1,000s level until the mid 1810s. The population remained stable until the beginning of World War I, the largest population that the census recorded was 1,159 in 1831.