Complex sentence: Due to the loss of Europeans from the city, many were killed, the rest flew back to France, the Cathedral de Sacre Coeur was converted into a public library for the city's predominantly Muslim residents, in order to make best use of the building.
Split into simpler sentences: Due to the loss of Europeans from the city, many were killed, with the the rest flew back to France. The Cathedral de Sacre Coeur was converted into a public library for the city's predominantly Muslim residents, in order to make best use of the building.