Germany declared war on the United States.
Iran declares war on the US every day. And you didnt answer the question.
We dont need to go around and police the world, but history shows that when threats do exist, like illnesses, its best to treat early rather that let them metastasize to where expensive and risky surgery is required.
The US hung out of Europe in WWII until Pearl Harbor, by which time Hitler controlled the European continent. Lots of what if’s, but what if the Allies had intervened sooner? Might have save millions of lives, including fewer losses of US soldiers.
Please dont be myopic when citing history.