Answer: It helped bring a quick victory to the Allies.
The United States had been providing some support to the Allies prior to its own entry into the war. But when the US joined the effort fully with a declaration of war, that not only meant the arrival of fresh US troops but a commitment of even more American dollars to the Allies' war effort. The American commitment greatly boosted the morale of British and French troops on the Western front and demoralized the German troops.
As to other answers I've seen suggested with this question in multiple choice format:
Russia did pull out of the war, but that was because it went through a revolution and change of government in its own country, not because of American involvement. Russia's fight was with Germany on the Eastern front, whereas the US entered the war on the Western front.
The United States entered the war on the Western front where the British and French had already been fighting the Germans.
There were still additional casualties after the Americans entered the war, but bringing the war to an end sooner cut down on the huge losses of life that had been happening.
"The South had more immigrants in large cities" was not a reason for the growing division between North and South after 1820, since the North had far more immigrants.
Belief systems are also known as religions. Religion has a big impact on human behavior. Belief systems influence how we live our lives, treat others, and should only influence human kind positively.
