Although there are many pivotal moments in the war, I'll actually start with the failed pre-war diplomatic efforts to keep the war from happening in the first place. Had any of the primary actors involved been able to stay the movement of troops to the front, the war might have been averted, even with the many secret alliances in place in both the Triple Entente and the Triple Alliance.
Another pivotal moment I should point out is the movement of German troops through Belgium during the first offensive of the war on their way to Paris during the "Battle of the Frontiers". Until the German troop columns actually passed within striking distance of the Belgian artillery and came under fire, the assumption of the German General Staff was that the Belgians would simply allow their forces to march through and fight a quick war with France, then return home. Belgium's response to this act of aggression slowed down the German advance and prevented the General Staff from out-flanking the French army at the Battle of the Marne river and from ending the war quickly.