Because the USA was supplying Great Britain, and put Great Britain in debt. Since USA wanted their money back, they know the English had to win, so we entered WWI. Also, we wanted a say in the Peace Treaty Conference to basically reshape Europe.
As the US expanded, the states brought into the country had to choose whether or not they would allow slavery, forcing them to take sides in the Civil War.
The good guys (Us) and the bad guys(Them)
USSR and the US in the Cold War and Nuclear Arms Race
Once Germany surrendered, it was the US against Japan.
Total cost estimated in US currency is $306,000,000.
Because they didn't feel they had to and because WW1 had lowered there supplies
a. The government and trade industries were selling weapons to all belligerents during the war. The president was telling the people to not choose sides and to stay neutral during the war. Also the use of propaganda!
Which war?
Not all of the US States had declared their loyalties before and after the US Civil War began. These States are referred to as the "Border States". Kentucky, as one example declared neutrality as one of the border States.
There was Mexico and the US.
The Cold War.
The entry of the US with fresh troops when the troops of both original sides were already exhausted.