The Great World War (World War I) began in August of 1914, but the United States did not enter the conflict until 6 April 1917. The Armistice was signed in 1918 at the eleventh hour, eleventh day of the eleventh month. Thus, we celebrate Veteran's Day on November 11th.
While the war did not last as long as many others, there were still American who were involved in the war including Nebraskans. There was nothing great about the Great World War which has been followed by many other wars and conflicts. Soldiers lost their lives, were dismembered or came home suffering from gas poisoning.
The Nebraska History & Record of Pioneer Days Volume II No. I (part 2) contains a listing of Nebraska's Dead during World War I. It can easily be searched online. The facts are also there. There were 349 killed in action; 207 died of disease; 160 died of wounds; 19 reported dead in an accident; 16 reported dead. This makes a total of 751 soldiers from Nebraska.
Nebraska Alumni Killed in WWI as taken from The Cornhusker, 1919, Vol. 13, University of Nebraska is also online. This contains a list of names with links to further information and photographs.
There are draft registration cards available on such in such places as Ancestry.com. However, you will also find draft lists published in newspapers or draft cards for Nebraskans at Nebraskans in the Great World War (WWI).
If your ancestor survived World War I, he may be shown on the 1930 U.S. Census with WW in one of the boxes. Keep in mind that men registered but never served.