My grandmother’s great-grandparents were on their way from Italy to Argentina when they took a stopover in Barcelona. Until recently, mine was the only branch of that family to have Spanish nationality: my grandmother’s grandfather, uncles and cousins all chose Italian nationality precisely as a way to avoid the draft.
If they’d chosen Spanish nationality, they would have been due 18-24 months military service.
By being Italian, all they had to do was go to the Consulate periodically to establish that they were still living in Spain. I understand that once this went on for several years, they could be called for “reduced service” which would have meant some work at the consulate (coffee gopher?), but the most any of them ever had to serve was one month.
My great-grandfather chose Spanish nationality. In theory, because he was a Catalanist and wanted to be able to vote when the issue of Catalonia becoming an independent country came up for it. In reality, I suspect that being “unable to serve” due to Coke-bottle glasses helped his decision
Now Spain doesn’t have a draft any more and by being Italians they actually have more obligations than our common ancestors did (new treaties between the two countries), so I understand that some of my distant cousins may have chosen Spanish nationality.