Suleiman devoted many years to the extension of his power in Europe, sometimes in alliance with the French king, sometimes upon his own initiative,--and with almost unbroken success. In 1521 he declared war against the king of Hungary on the pretext that he had received no Hungarian congratulations on his accession to the throne. He besieged and captured Belgrade, and in 1526 on the field of Mohács his forces met and overwhelmed the Hungarians, whose king was killed with the flower of the Hungarian chivalry. The battle of Mohács marked the extinction of an independent and united Hungarian state; Ferdinand of Habsburg, brother of Charles V, claimed the kingdom; Suleiman was in actual possession of fully a third of it. The sultan's army carried the war into Austria and in 1529 bombarded and invested Vienna, but so valiant was the resistance offered that after three weeks the siege was abandoned. Twelve years later the greater part of Hungary, including the city of Budapest, became a Turkish province, and in many places churches were turned into mosques. In 1547 Charles V and Ferdinand were compelled to recognize the Turkish conquests in Hungary, and the latter agreed to pay the sultan an annual tribute of 30,000 ducats. Suleiman not only thwarted every attempt of his rivals to recover their territories, but remained throughout his life a constant menace to the security of the hereditary dominions of the Habsburgs. [Sidenote: Charles V and the Holy Roman Empire.] [Sidenote: Possibility of transforming the Empire into a National German Monarchy]