Explanation: Roman society was remarkably open, by the standards of ancient societies, to outsiders. Most societies only allowed full membership in the polity for those with the proper ethnic descent. The Romans, while automatically granting citizenship to the children of any two citizens, also freely granted citizenship to any who proved themselves sufficiently loyal and useful to the res publica. This could take many forms - grants of citizenship were used to reward cities which had been loyal for a sufficiently long time, or to lure local elites into supporting the Roman government, or as a gift for exceptional deeds (including, curiously, a slave-turned-freedman who became a baker and was granted citizenship simply for supplying bread to the city of Rome for a number of years).
However, the most reliably attested-to method was instituted by the Roman Emperor Claudius, who formalized that any provincial, freedman, or barbarian who served 25 years in the Roman auxiliary forces, in support of the famous Roman Legions, would receive citizenship for themselves, their wife, and their children. Service, clearly, guarantees citizenship!
The drawing is a reference to the movie Starship Troopers, a satire of jingoism and imperialism, but 'service guarantees citizenship' comes from the original book, Starship Troopers, which, depending on who you ask, is either pseudofascist drivel or an interesting exploration of what a hypermilitarized sci-fi democracy might look like.
