Guerra made his unofficial debut for San Marino while playing for ASD Tropical Coriano in a 1987 Mediterranean Games 0–0 draw with Lebanon on 16 September. His official debut came in a FIFA-sanctioned 4–0 loss to Switzerland in a Euro 1992 qualifier on 13 November 1990. Guerra went on to represent San Marino in qualifiers for the 1994 FIFA World Cup, UEFA Euro 1996, 1998 FIFA World Cup and UEFA Euro 2000. He captained his country for the first time in the 7–0 home loss to Russia on 7 June 1995. He was also captain in the games against Greece, in September, and the Faroe Islands in October. Guerra then had to wait until October 1996 to regain the armband, when San Marino played Belgium. Guerra's final game as captain came in a 6–0 loss to Belgium in June 1997.