These three brothers served in Europe during the war. None appeared to be career military before or after the war. So each Flanagan brother should have registered for the draft. Try as I might, I could not find a draft registration card for the oldest brother Michael Flanagan.
Before you begin digging into Mexican records, you need to first narrow down the scope of your search by identifying you ancestor’s hometown in Mexico. The reason for this is twofold: it narrows down the focus of your search, and it helps you identify the right collections to use.