Which factor play a major role in the decline of the Ottoman Empire

Foreign militaries began developing more advanced weaponry than the Ottoman military.

The incapability of the Sultans was one of the most important factors that caused the decline of the Ottoman Empire. The slow and steady decline of the Ottoman Empire began in the reign of Süleyman I. By spending much of his time in campaigning and strenuous functions of administration withdrew himself from public affairs and dedicated himself to the joys of his harem. In order to take his position, the post of grand vizier was formed up to become second only to the sultan in government and taxation; the grand vizier's power involved the right to demand and get authoritarian obedience. Though the grand vizier was capable of standing in for sultan in the official roles, he could not get the loyalty of different people of the empire. In this way, the separation of central authority and political loyalty led to a decline in the power of government to impose it well.