Is it permissible to appoint Christians to leadership positions in Islamic lands?

The Salaf (pious predecessors) used the following verses as evidence to prohibit seeking the assistance of disbelievers in positions of authority:

Allah's saying: "O you who have believed, do not take the Jews and the Christians as allies. They are [in fact] allies of one another."

And His saying: "Your ally is none but Allah and [therefore] His Messenger and those who have believed – those who establish prayer and give zakah, and they bow [in worship]. And whoever takes Allah and His Messenger and those who have believed as allies – indeed, the party of Allah – they will be the predominant."

And His saying: "And those who disbelieved – some of them are allies of others."

Imam Ahmad narrated with an authentic chain of narration from Abu Musa that he said to Umar: "I have a Christian scribe." Umar said: "What is wrong with you? May Allah fight you! Did you not hear Allah say: 'O you who have believed, do not take the Jews and the Christians as allies. They are [in fact] allies of one another.' Why did you not take a Hanif (monotheist)?" Abu Musa said: "O Commander of the Faithful, his writing is for me, and his religion is for him." Umar said: "I will not honor them when Allah has disgraced them, nor will I exalt them when Allah has humbled them, nor will I bring them near when Allah has distanced them."

This indicates that appointing disbelievers from all religions and sects to leadership positions is a forbidden matter, agreed upon by the imams of the Muslims.