A political alliance between Islam and Christianity would be a...

A political alliance between Islam and Christianity would be a show of great common weakness vis-à-vis secularism or a rapid expanding new religion. They are incompatible and in competition.

"in classical Islamic law, the major divisions of the world in Islam are dar al-Islam (lit. territory of Islam/voluntary submission to God), denoting regions where Islamic law prevails,[1] dar al-sulh (lit. territory of treaty) denoting non-Islamic lands which have concluded an armistice with a Muslim government,[2] and dar al-harb (lit. territory of war), denoting adjoining non-Islamic lands whose rulers are called upon to accept Islam.[3]...

Under the classical doctrine, it was the duty of Muslim rulers to bring dar al-harb under Islamic sovereignty...

A state of war was presumed between dar al-harb and dar al-Islam..."