Despite President Wilson’s efforts during his first term in office, and the initial restriction of German submarine warfare after the Lusitania, in January 1917 Germany determined the U.S.A. would eventually enter the war and so offered Mexico funding if they would attack their neighbour, and re-engaged in unrestricted submarine attacks. The U.S.A. declared war in April, 1917.
One of the first things the government did was call for the registration of all able-bodied men, in preparation for a draft. On June 20, 1917, the Warren Sheaf announced on the front page (if below the fold) that nearly every person required had registered. But they went further than this: they listed, on page three (pdf), the complete selective service registry for Marshall county, by township name. You may have an ancestor listed here!
As soon as possible we will have a transcription of that list online, hopefully in a searchable format. The National Archives and Records Administration[en.WP] has microfilm of all these selective service records which include such data items as date and place of birth, current residence and employment, and a general physical description of the registree. If we can locate an online source for copies of these records, we will link them together.