 Complete the following phrase:  Meanwhile, the Ottoman issue had again become acute, and, after the fall of Constantinople in 1453, Sultan Mehmed II was rallying his resources to subjugate Hungary. His immediate objective was Nándorfehérvár today Belgrade. Nándorfehérvár was a major castle-fortress, and a gatekeeper of south Hungary. The fall of this stronghold would have opened a clear way to the heart of Central Europe. Hunyadi arrived at the 