World War I started because of entangling alliances in Europe which eventually spread to our allies, France, England. The U.S. tried to stay out of it until the Lusitania was sunk by a German U-Boat
World War II started with the Hitler led takeover of Eastern Europe, which eventually brought us in as Allies to England and France. That War had a second Front in the Pacific and started with the Japanese bombed Pearl Harbor. The Japanese was opposed to U.S. Presence in the Pacific.
Those are the short answers.
The shorter answers is that in every case dictators who cared little about others started the wars. There are thousands of reasons, economic, ethnic, social, religious, etc behind each that as already noted in another post are too broad for this forum.
In each of those wars, there was a direct threat to the United States. Later wars, Korea, Vietnam, and the current battles did not pose such a clearly defined threat to the United States.