Queen Mary of Hungary in Pozsony) and the Pope offered 50,000 gold pieces, while neither Charles V nor Ferdinand Habsburg (Archduke of Austria, the Hungarian king's brother-in-law) did anything. The fact is that the Habsburgs' armies were still on the battlefields of Italy. Preparations The Hungarians had long opposed Ottoman expansion in southeastern Europe, but in 1521 the Turks advanced up the Danube River and took Nándorfehérvár (present-day Belgrade, Serbia) – the strongest Hungarian fortress on the Danube – and Szabács (now Šabac, Serbia). This left most of southern Hungary indefensible. The loss of Nándorfehérvár caused great alarm in Hungary, but the huge 60,000 strong royal army – led by the king, but recruited too late and too slowly – neglected to take food along. Therefore, the army disbanded spontaneously under pressure from hunger and disease without even trying to recapture Belgrade from the newly installed Turkish garrisons. In 1523, Archbishop Pál Tomori, a valiant priest-soldier, was made Captain of Southern Hungary. The general apathy that had characterized the country forced him to lean on his own bishopric revenues when he started to repair and reinforce the second line of Hungary's border defense system. Pétervárad fell to the Turks on July 15, 1526, due to the chronic lack of castle garrisons. The Danube river was an extremely important transport route for the Ottoman army in the region, so it was clear to everyone that the Ottoman army would follow the line of the Danube. For about along the Danube between Pétervárad and Buda there was no single Hungarian town, village, or fortification of any sort. Three years later, an Ottoman army set out from Constantinople on 16 April 1526, led by Suleiman the Magnificent personally. The Hungarian nobles, who still did not realize the magnitude of the approaching danger, did not immediately heed their King's call for troops. Eventually, the Hungarians assembled in three main units: the Transylvanian army under John Zápolya, charged with guarding the passes in the Transylvanian Alps, with between 8,000 and 13,000 men; the main army, led by Louis himself (beside numerous Spanish, German, Czech, and Serbian mercenaries); and another smaller force, commanded by the Croatian count Christoph Frankopan, numbering around 5,000 men. The Ottomans deployed the largest field artillery of the era, comprising some 300 cannons, while the Hungarians had only 85 cannons, though even this number was greater than other contemporary Western European armies