This is copied and pasted from the Ancestry.com website explaining the WWI Draft Registration Cards which they have online images of:
On 6 April 1917, the United States declared war on Germany and officially entered World War I. Six weeks later, on 18 May 1917, the Selective Service Act was passed, which authorized the president to increase the military establishment of the United States. As a result, every male living within the United States between the ages of eighteen and forty-five was required to register for the draft.
The period of 1880-1920 was a high immigration period to the United States. Young men were required to register for the draft regardless of their U.S. citizenship status. Of course, not all the men who registered actually served in the armed forces, and there were some who served in the war but did not register for the draft."
So having a Draft Registration Card or "paper" does not mean one was "drafted". In very simple terms they took the cards and then the Selective Service Office held a lottery to pick those who would be sent a notice of conscription into the military service. If a man was already in the service he was exempted from filing a card.
All U.S. Military service records are stored at the NARA
) In 1973 they has a fire that destroyed most of all Army personel records from WWI to 1960's. Other services at the time had their records stored elsewhere, but today all services are stored there. You might get lucky and your great uncle(s) might only be singled. The website has directions and forms to download for a search.
At the American Battle Monuments Commission website has an index of U.S. Service men buried overseas from WWI. http://www.abmc.gov/home.php
Remember there was an 1918 flu pandemic that killed an estimated 43000 U. S. service men of WWI.
Local papers would publish the names of those in their area that were serving, on leave visiting family and deaths. Also War Memorials were made listing those who died from the county, city or state that wanted them to be remembered.
What were their names and I can check my online sources for clues. Where were they living at the time? The 1930 census asked a question about being a Veteran and what war. Do you know if the 45 yr old was still alive for this census?