Complex sentence: Members of both branches of the military supported both sides; however, with the military setbacks at Nomonhan and the Mongolian front in the late 1930s, many Army supporters of the "Strike North Doctrine" changed sides.
Split into simpler sentences: Members of both branches of the military supported both sides. After the military setbacks at Nomonhan and the Mongolian front in the late 1930s, many Army supporters of the "Strike North Doctrine" changed sides.