Because real life isn't a video game. There are millions of people in those city-states, the countries have important diplomatic relations, and many other things. Look at the huge backlash when Russia annexed Crimea, land that was inhabited by Russians and was part of it 70 years ago. Imagine then if France annexed Monaco, a more populated, richer and western country.

This submission has been removed because it violates our '20-Year Rule'. To discourage off-topic discussions of current events, questions, answers and all other comments must be confined to events that happened 20 years ago or more.

Sorry, but your submission has been removed because we don't allow hypothetical questions. If possible, please feel free to rephrase the question so that it does not call for such speculation, and resubmit Otherwise, this sort of thing is better suited for /r/HistoryWhatIf.