Recently someone explained me that San Marino fought (although with a small army) next to Italy (Allied Powers) in WW1 but treaties of Versailles, Trianon, Lausanne etc... never included San Marino as a signatory. (Because it was forgotten?) So, San Marino was de jure at war with Germany, Austria, Turkey etc... after WW1. Is this claim true?
No. Ten volunteers and a handful of medical personnel from San Marino fought alongside the Italians, but the country itself remained neutral.