Due to a genetic quirk - my Luxembourger ancestor had girls - my family wasn't of age to serve any of the 19th century conflicts. They immigrated too late for the Civil War, were too old for the Spanish-American War, and had female children during the Spanish-American War. It wasn't until I spotted my ancestor's cousin on the Spanish-American War pension that I realized this was true only of my family!
Don't forget to time out your Luxembourger ancestors birth dates and look for military information. If they immigrated in the 1850s with small children, those ancestors' grandchildren might have been of age to serve in the Spanish-American war. If they were between 18-25, chances are high. If you have access to Fold3.com, check out their Civil War and later pension index. It will give you a hint as to whether to continue searching.