Is it permissible to pledge allegiance to a disbelieving ruler?

The Bay'ah (pledge of allegiance) is a legitimate contract between the one pledging allegiance and the Amir (leader) or Caliph, signifying a covenant of obedience. The Bay'ah is concluded after the Ahl al-Hall wal-'Aqd (people of authority and influence) choose the Caliph. These are people of knowledge, sound judgment, and intellect, who possess the conditions of trustworthiness and justice.

It is a condition for the Caliph to whom allegiance is pledged that he be a Muslim by consensus, because he is the one who will implement Allah's law, establish the hudud (penalties), and guard the frontiers. If the Caliph commits an act of disbelief (kufr), he is to be deposed.

Allah Almighty says: "And never will Allah grant to the disbelievers a way (to triumph) over the believers" (An-Nisa 4:141). Therefore, it is not permissible to pledge allegiance to a disbelieving ruler.