In the 1800s, the Ottoman Empire was becoming much weaker than it had been. One of the major reasons for this was nationalism. The Ottomans ruled many national groups who aspired to have their own states.
As this happened, tensions arose in Europe as various countries hoped to gain from the disintegration of the Ottoman Empire. Russia seemed best situated to benefit from the disintegration but other countries like Austria, France, and Britain hoped to gain as well. These countries also worried about the possible increase in Russian power that might come about if Russia gained too much from the Ottoman Empire.
These sorts of concerns led to greater tension and less stability in Europe. The prime example of this was the Crimean War.