How did nationalism within the Ottoman empire come into play during the war?WWI
The main way that this came into play was in how the Allies tried to exploit ethnic differences within the Ottoman Empire. Perhaps the most famous name associated with this process is Lawrence of Arabia.
The Ottoman Empire ruled over large numbers of people who were not ethnically Turks (the ethnicity of the rulers). During World War I, the Allies tried to foment rebellions among the Arabs, for example, to weaken the Ottoman Empire.
The other major impact was the fact that Turkish nationalism helped cause what some call the Armenian genocide. Ethnic Turks came to be suspicious of Armenians are killed large numbers of them.