Charles Sturt University lists three major turning points in World War I. One was when the United States entered the war in 1917. The second was when Russia left the war in 1917 and the final turning point listed involves Ludendorff's final offensive in 1918.
America declared war on Germany in 1917 after President Wilson had unsuccessfully encouraged peace talks. The United States needed to protect financial and commercial interests. America gave more funds and support to Great Britain and France while drastically decreasing trade and loans with Germany at the same time.
Also in 1917, Russia withdrew from the war because of low morale. Russians were running out of food and supplies, resulting in revolts. Though the Russian government attempted to continue on with the war efforts originally, when Lenin's Bolsheviks stormed the Winter Palace, they ordered the cessation of hostilities.
Germany had a breakthrough in 1918 when the troops under the command of Erich Ludendorff attacked the British and French army juncture. Unfortunately, this caused supply problems for the Germans, weakened their flanks and reduced the rate of replacements. The French and British decided to combine forces to create a counter-attack. Using tanks, the new combined force was able to cause over 40,000 casualties, forcing the Germans to retreat. It was after this point that Ludendorff advised the German government to sue for peace.