How the U.S. & Russia Changed WWI

    The United States joined World War I with the Allies on April 2, 1917 because the Germans said their submarines would begin sinking any ship near England. 3 of the U.S. Ships were sank which then angered the Americans.This plan was called unrestricted submarine warfare. Also Germany said would help Mexico "reconquer" land it had lost to the United States if Mexico would team up with Germany.

One of the reasons Russia quit World War 1 is because in 1917 nearly 5.5 million Russian soldiers had been wounded, killed, or taken prisoner. Also they suffered a lot which made Russians refused to fight any longer.

Germany felt confident about winning World War I because they mounted one final, massive attack on the Allies in France. Germany destroy everything in their path. Once they reach Marne river Paris was less then 40 mile of their reach. Victory for Germany and The Central Powers seemed within their reach. 
US made a significant impact on the war stopping Germany in France, and destroyed their hope to win the war, losing massive amounts of troops.
Three significant impacts the war had on the world was the world lost 8.5 million soldiers, many civilian's suffered and died of starvation diseases and slaughters, and every continent in the world was involved.