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1. Became independent in 301 AD?
From the Roman Empire.
Monaco
San Marino
Both
2. Borders the Mediterranean Sea?
San Marino is landlocked by Italy.
Monaco
San Marino
Both
3. Has a monarchy?
Monaco
San Marino
Both
4. Has a national football team which is a member of FIFA and UEFA?
Monaco has a national football team which has not joined FIFA or UEFA and therefore does not in participate in major international tournaments.
Monaco
San Marino
Both
5. Has a population lower than 50,000?
Monaco has 38,300 inhabitants and San Marino has 33,344.
Monaco
San Marino
Both
6. Has never won a medal at the Olympic Games?
...despite both competing.
Monaco
San Marino
Both
7. Is not a member of the EU?
Monaco
San Marino
Both
8. Has a GDP per capita of over 180,000 US dollars?
Monaco
San Marino
Both
9. Is the least visited country in Europe?
San Marino received 84,000 visitors in 2018 whereas Monaco received 347,000 according to UNWTO: https://bit.ly/3gW47t2
Monaco
San Marino
Both
10. Has won the Eurovision Song Contest?
Monaco won in 1971 with the song "Un banc, un arbre, une rue" by Séverine. They declined to host the following year.
Monaco
San Marino
Both
11. Relies on another country for its defence?
Monaco relies on France and San Marino on Italy.
Monaco
San Marino
Both
12. Bans its citizens from gambling?
Although famous for gambling, Monaco only allows this for tourists and foreign residents.
Monaco
San Marino
Both
13. Shares its name with its capital city?
San Marino City is the capital of San Marino, Monaco is a city-state and is its own capital.
Monaco
San Marino
Both
14. Uses the euro?
As per agreement with the Council of the European Union.
Monaco
San Marino
Both
15. Made Abraham Lincoln an honorary citizen?
In 1861, Abraham Lincoln sent a letter to San Marino saying, "Although your dominion is small, nevertheless your State is one of the most honoured throughout history…". In response, they granted him citizenship.