Complex sentence: However, war-weariness was a major factor by 1917, even reaching the army, as soldiers were reluctant to attack--many threatened to mutiny--saying it was best to wait for the arrival of millions of Americans.
Split into simpler sentences: However, war-weariness was a major factor by 1917, even reaching the army. Mutiny was a factor as soldiers said it was best to wait for the arrival of millions of Americans.