What are the given names of your SEGINA? What are their approximate birth years. Where in the US did they settle. Also Ellis Island (port of NY) was not the only port of entry used during that time... as I have access to the others perhaps I can help.
If they came in the early 1900s then it is unlikely he fought in any war. WW I did not begin until 1914 and things were relatively calm for several decades prior to that.
Croatian Heritage Museum