What was the name of Charlemagne's theological work against iconoclasm?

Charlemagne’s theological work against iconoclasm was the Libri Carolini, also called Opus Caroli regis contra synodum. The four-book work was composed in the mid-790s at Charlemagne’s command to challenge the conclusions of the Byzantine Second Council of Nicaea, held in 787.

The controversy concerned religious images. The council defended the veneration of icons while distinguishing that honor from the worship owed to God alone. The Libri Carolini rejected the council’s position, arguing that images could serve as decoration, instruction, and reminders of sacred history, but should not receive prayer, incense, or divine reverence.

Charlemagne probably did not write the work himself. Modern scholarship commonly connects its main composition with Theodulf, a learned court theologian who later became bishop of Orléans. The text was not simply a call to destroy all religious art: its more precise position opposed treating images as objects of worship while still allowing them useful roles in Christian teaching.