Complex sentence: From 1943, Japanese troops suffered from a shortage of supplies; especially food, medicine, munitions and armaments largely due to submarine interdiction of supplies and losses to Japanese shipping, which was worsened by a long-standing and severe rivalry with the Imperial Japanese Navy.
Split into simpler sentences: From 1943, Japanese troops suffered from a shortage of supplies; especially food, medicine, munitions and armaments largely due to submarine interdiction of supplies and losses to Japanese shipping. Which was worsened by a long-standing and severe rivalry with the Imperial Japanese Navy.