When US entered WWI, what century would that be? 20th century?
20th century, yes -- 1914.
Sorry -- I think the US entered in 1915.
The U.S. entered WWI in 1917.
Yes, when the United States entered World War I, it was in the 20th century. To determine this, we can follow a straightforward approach of understanding the historical context.
World War I, also known as the Great War, began in 1914 and ended in 1918. The 20th century spans from the year 1901 to 2000. Since the United States entered the war after it started in 1914, we can conclude that its involvement was also within the 20th century.