What is the Islamic legal directive regarding the Ottoman Caliphate? Was obedience to them obligatory? And are they sinful for establishing it, given that the Prophet (peace be upon him) commanded that Qurayshis be appointed as leaders?

It has been previously explained that the condition of belonging to the Quraysh for the Grand Imamate applies when the Imam is chosen. However, if the Imam assumes the Imamate through other means, such as by force (al-mutaghallib), then the condition of belonging to the Quraysh is not required, and obedience to him becomes obligatory as long as he does not command disobedience.

Sheikh Ibn Uthaymeen was asked about reconciling the hadith: "You must listen and obey, even if an Abyssinian slave is appointed over you," with the hadith: "The Imams are from Quraysh." He was also asked whether it is permissible for an Abyssinian slave to be a Grand Imam. He replied that the hadith "The Imams are from Quraysh" is specific to the case of choice. If someone gains power over people and rules them, then listening and obeying him becomes obligatory, even if he were an Abyssinian slave.

Ibn Uthaymeen added that there is no Grand Imam in our time, and the general Imamate has been absent since the era of the Companions. So, whoever takes control of a piece of land and becomes its ruler is an Imam.

Regarding the title of Caliphate for the Ottoman Empire, he mentioned that the Ottoman princes did not adopt the title of Caliphate until the last Abbasid caliph relinquished it to Sultan Selim. However, some sources indicate that Sultan Selim had called himself "Caliph of Allah" before that, and that the title of Caliphate was not a primary concern for the Ottomans until their state weakened. Regardless, the authority of the Ottomans was legitimate, and they defended Islam and Muslims.