The Hungarians had long been opponents to the Muslims, the "Shield of Christianity" for Europe. The marriage of Louis to Maria of Austria in 1522 drew the kingdom closer to the Habsburgs and the Ottomans saw the need to break this nascent alliance, after Louis refused a peace offer the Ottomans decided to use military power. They attacked in June 1526, advancing up the Danube.
The Hungarian forces chose the battlefield, an open if uneven plain leading down to the river Danube. The Ottomans had been allowed to advance almost unopposed. While Louis waited in Buda, they had besieged several towns and crossed the Sava and the Drava. Louis had around 26,000 soldiers and the invading army was around 50,000 - 60,000. The Hungarian army was arrayed to take advantage of the terrain and hoped to engage the Ottoman army piecemeal.
The actual battle lasted only two hours. As the first of Suleiman's troops, the Rumelian army, advanced onto the battlefield at 13.00 they were attacked and routed by Hungarian troops led by Pal Tomori. But as the main Ottoman force arrived in the early afternoon (around 14.00) the situation quickly changed, the Hungarian forces were slow to reinforce the successes on their right and when they did advance, too late, they became exposed. Nevertheless the Hungarian soldiers fought bravely, but decimated by Ottoman musket fire they could not last and either fled or were surrounded and killed or captured. Louis left the battlefield but was thrown from his horse in a river and died there. Around 16,000 Hungarian soldiers were killed and a similar number of Ottomans.
The Ottoman victory did not give them the security they wanted. It meant the end of the Kingdom of Hungary but the Ottoman forces withdrew in September and the territory ended up split between the Ottomans and the Hapsburg Ferdinand, Archduke of Austria.