The Motor Boys in The Army is the 19th volume in the series. It was published sometime during 1918. War looks likely, so it should come as no surprise that the boys don't hesitate a second signing up. Well, Bob hesitates a bit, but it's not because he's scared or doesn't believe in the war. In fact, World War I began July 18, 1914, and ended November 11, 1918. This means that if the war wasn't over when Young was writing the book, it's outcome was almost certain, since the U.S. entered the conflict April 6, 1917, and shortly thereafter, American soldiers began arriving on the Western Front in Europe and joined the battle at the rate of 10,000 per day. This infusion of additional fighting men was the major reason the balance quickly shifted to the Allied side of the conflict. The boys enter the fray in the next volume.
Clarence Young was a Stratemeyer Syndicate pen name. Howard Garis almost certainly wrote the series, although Stratemeyer usually provided his authors with an outline of the books. Garis and his wife, Lillian, wrote many children's books under their own name. They are best known for a series of books that featured the character of Uncle Wiggily Longears, an engaging elderly rabbit. Many of Garis' books were illustrated by Lansing Campbell. Garis and his wife, Lilian Garis, were possibly the most prolific children's authors of the early 20th century. Garis wrote many books for the Stratemeyers Syndicate as well, over a period of some 30 years.