Complex sentence: At that time, the FTP armed wing already was active since 1941, but the Resistance quickly expanded itself in 1942-1943, French population's morale getting higher with the difficulties faced by the Wehrmacht, in particular during the 1942-43 Battle of Stalingrad.
Split into simpler sentences: At that time, the FTP armed wing had already been active since 1941, but the Resistance quickly expanded itself during 1942 and 1943. The French population's morale improved as the difficulties faced by the Wehrmacht increased, in particular during the protracted Battle of Stalingrad.