The death spasms of the ancient Ottoman Empire were a major factor in precipitating World War One. That empire was dying, and much of Europe wanted a piece. Britain had grabbed Egypt, France had gobbled up several North African territories, and Russia and Austria Hungary had major ambitions in Europe. Germany wanted a piece of the African action, and so did Italy, but the most fraught struggle was in the Balkans.
The major powers were not the only ones engaged in trying to gnaw off pieces of the not yet dead Ottoman corpse. Various Balkan provinces had briefly united to throw off Ottoman rule, but promptly turned on each other afterwards, each trying to grab more of the common pie. Hundreds of years of imperial rule had shaken, stirred, and mixed the various ethnicities, but rising nationalist sentiment everywhere was undoing the mix. Then, as now, every tinpot imperialist could claim to be intervening to protect their fellow Christians, Catholics, Orthodox, fellow Slavs, Serbs or whatever.
The most powerful and aggressive Balkan country was Serbia, and Serbia's Russian ambassador was inciting its extremists even while his bosses, the Foreign Minister and the Tsar were urging them to dial it back. Russia's big stake was preservation of its access through the straights connecting the Black Sea to the Mediterranean and the world. Austria Hungary feared Serbia, which was already agitating among the Slavs of its Empire, and desperately did not want it to get territory and a seaport on the Adriatic.
Again and again these conflicts brought Europe to the brink, until the Serbian murder of the Archduke and heir to the Austro Hungarian monarchy pushed it over.