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Links to similar answered questions:
Why hasn't Italy ever conquered San Marino?
Why has nobody ever bothered to annex San Marino in over 1700 years?
San Marino was founded in 301 AD. How did it survive all the political instability that reigned throughout Europe in the near 1700 years since it was founded?
How have the micro/small states of Europe (Andorra, Liechtenstein, Luxembourg, Monaco, San Marino) remained sovereign states for so long?
Why did the Papal States never annex San Marino?