The US could no longer remain neutral in WWI because of Germany's policy of unrestricted submarine warfare and the Zimmerman note. Also, the United States had economic and cultural ties to the allies.
After WWII, and before the invasion of Pearl Harbor the US committed to remain neutral toward conflicts in Europe because they want to be a peaceful nation. In addition, the American people have voiced there opinion asking the US to remain neutral on conflicts in Europe.
yes
the first twenty years of the constitution
German submarines were attacking all ships near Europe, even those from neutral nations or carrying passengers from neutral nations. The US also discovered that Germany was trying to persuade Mexico to join the Central Powers and attack the US to regain land Mexico lost in wars against the US.
neutral
The US decided to remain neutral in this war, however this pact did not last long.
The US wasn't attacked (yet).
Most of the time, the US likes to take action.
The Proclamation of Neutrality
Technically, four so-called border States were neutral in the US Civil War. They were Missouri, Kentucky, Maryland and Delaware.
America tried to remain neutral while still supporting the allied nations
The attack on Pearl Harbor