The fourth year of fighting brought several of the combatants to the point
of collapse. France, Italy, and Russia discovered that their military forces
were no longer stable organizations. Events on the home front now took on
crucial significance, most notably in Russia, where the governing system
itself fell victim to the strains of the fighting.
At the same time, older features of the war continued. The war spread to
bring in new belligerents, the crucial newcomer being the United States.
Moreover, the massive and seemingly futile bloodletting on the western
front did not abate; the carnage took its most hideous form at the Third
Battle of Ypres (Passchendaele).