[1] After the death of Victor's successor, Paschal III, on 20 September 1168, his supporters gathered in Rome to elect a new (anti-)pope.
[2] According to Cardinal Boso of Santa Pudenziana, a partisan of Alexander III, Frederick was "inwardly deeply grieved" by the election of Callixtus, whom he had never met.
The antipope was primarily a bargaining chip with which the emperor could pressure Alexander, and had only limited geographical support in comparison to Victor IV and Paschal III.
[2] In 1173, Callixtus sent Martin, cardinal-bishop of Tusculum, to Germany as legate to help mediate an alliance between Frederick and King Louis VII of France.
In November 1176, he signed the Pact of Anagni, which did, however, protect Callixtus by granting him an abbacy and restoring the cardinals he had created to their former offices.
[1][2] Callixtus did not immediately recognize the Peace of Venice, but in 1178 Archbishop Christian of Mainz, the imperial chancellor, marched an army into Italy to retake the Papal State and Rome for Alexander.
[2] Some of his obstinate partisans sought to substitute a new antipope, and chose one Lando di Sezze under the name of Innocent III.