and functioning city. During their earliest exploits, the Ottomans were frequently frustrated by fortifications and the exhausting sieges that were required in order to overcome these defenders. This changed in the 15th century and beyond as imported expertise in firearms and gunpowder led to some of the most massive artillery of the time. A form of Turkish heavy cavalry, known as Sipahi, became the predominating military unit utilized by the Ottoman Empire. The earliest variations of these soldiers were well-armored men on well-armored horses, who typically used a mace as their primary weapon. During the 17th century the Sipahi replaced their archaic weaponry with sabers and pistols, establishing them as a fearsome presence on the battlefield. While European infantry were more than a match for the standard Ottoman infantry, the Sipahi were far superior to any medieval knights. During their period of expansion, the Ottomans visualized Europe as Americans would later see their Western frontier: a land of destiny. The conquest of Istanbul in 1453 AD initiated this by uniting Muslim Anatolia and Christian Rumelia under the Ottoman Sultan's protection. However, this attitude would change as the Ottomans were forced to go on the defensive during the 17th and 18th centuries. Ottoman defeats were followed by mass slaughters of Muslim minorities. These massacres characterized the war between Christian and Turk in the 19th century. At its height, the Ottoman Empire territory included Hungary, Serbia, Bosnia, Romania, Greece, the Ukraine, Iraq, Syria, Israel, Egypt, a large section of North Africa, and most of the Arabian Peninsula. Over time, however, Sultans grew weary of their administrative duties and withdrew from public affairs. The office of Grand Vizier was created to act second only to the sultan in authority and revenue, but while the Grand Vizier was able to stand in for the sultan in an official capacity, he could not take his place as a focus of loyalty among the different classes and social groups in the empire. This separation of political loyalty and centralized authority led to a decline in the government's ability to impose its will, which inevitably led to the Ottoman Empire's end in 1922, when it was replaced by the Turkish Republic.