The Ottoman Empire had fallen long ago. It had enjoyed an era of expansion, culminating in an attack on Constantinople itself under Sultan Mehmed. But that is where their triumph ended. The attack was a disastrous failure. The great cannon that was to be the end of Constantinople's walls exploded in a freak accident, and to make it worse, the Sultan was at the firing. He did not survive, and the loss of the Sultan forced a retreat from the city.
The next years were characterized by the ineptitude of the next Sultan. The Balkan territories were lost.