The United States signed a Treaty of Extradition with San Marino in 1906, and established consular relations in 1925, but did not appoint its first diplomatic ambassador to that country until November 22, 2006. Nonetheless, it could be argued that the United States recognized San Marino as early as 1861, when President Abraham Lincoln warmly accepted its offer of honorary citizenship.
Key Diplomatic Events
May 1861 was one of the darkest moments in the history of the United States, as the nation faced a civil war that, in the minds of many around the world, raised doubts about the viability of democratic republics. On May 7, 1861, President Lincoln addressed a letter to the Regent Captains of the Republic of San Marino:
Great and Good Friends
I have received and read with great sensibility the letter which as Regent Captains of the Republic of San Marino you addressed to me on the 29th of March last. I thank the Council of San Marino for the honor of citizenship they have conferred upon me.
Although your dominion is small, your State is nevertheless one of the most honored, in all history. It has by its experience demonstrated the truth, so full of encouragement to the friends of Humanity, that Government founded on Republican principles is capable of being so administered as to be secure and enduring.
You have kindly adverted to the trial through which this Republic is now passing. It is one of deep import. It involves the question whether a Representative republic, extended and aggrandized so much as to be safe against foreign enemies can save itself from the dangers of domestic faction. I have faith in a good result.
Wishing that your interesting State may endure and flourish forever, and that you may live long and enjoy the confidence and secure the gratitude of your fellow citizens, I pray God to have you in his holy keeping. Your Good Friend.
source: U.S. – San Marino Relations – U.S. State Department, Office of the Historian