When will a Khalifah of an Islamic state be a valid Khalifah according to the Shariah?

Answer

A Muslim ruler who enforces the Shariah strictly, is a legitimate Khalifah of Rasulullah (sallallahu alayhi wasallam) regardless of the method he has gained power, whether it be by nomination of an earlier ruler, or appointment by the senior Ulama, or by force of arms, or by a military coup. The requisite for a man to be a legitimate Khalifah is that he has to govern strictly according to the Shariah, not merely introducing some dimensions of the Shariah such as corporal punishment.

If the Khalifah is not a Quraishi, he may not assume the title of Ameerul Mu'mineen. Thus, even a great ruler such as Sultan Salaahuddin Ayyubi who held the actual reins of power during the declining days of the Abbaasi Khilaafate (Abbaside Dynasty), never proclaimed himself the Ameerul Mu'mineen. He was known as the 'Sultan'. He acknowledged the Abbaasi Khalifah in Baghdad as the Head (Ameerul Mu'mineen) of the Islamic Empire and ordered the reintroduction of the Khalifah's name for recitation in the Jumuah Khutbah – a practice which had been discontinued due to the abject weakness and emaciation of the rule of the Khalifah.