Complex sentence: Hindenburg and Ludendorff had demanded reinforcements from Falkenhayn to fight a decisive campaign against Russia and intrigued against Falkenhayn over his refusals on the grounds that decisive military victory against Russia was impossible, which made the Western Front the decisive theatre of the war.
Split into simpler sentences: Hindenburg and Ludendorff had demanded reinforcements from Falkenhayn to fight a decisive campaign against Russia and intrigued against Falkenhayn over his refusals. Falkenhayn held that decisive military victory against Russia was impossible and that the Western Front was the main theatre of the war.