Answer ↓
Approximately three years (from 1914 to 1917)
Explanation ↓
World War I began in 1914.
At first, the US claimed it was neutral (neutral means not involved in a war in any way) but the following reason caused the U.S. to join World War I.
- Germany resumed to attack passenger and merchant ships with submarines ( called U-boats)
The reason above was the main reason why the US joined World War I in 1917.
