What was the Bosnian Crisis of 1908 about?
In the early 1900s, the Ottoman Empire was weakening. The Young Turks were trying to modernize and strengthen it. Austria-Hungary jumped in and annexed Bosnia before the Ottomans could get strong enough to resist them. This led to the Bosnian Crisis.
The Austrians had a deal with Russia where Russia would allow Austria to take Bosnia and Austria would support Russia's desire to have the Dardanelles opened to their warships. But France and England would not agree to allow the Dardanelles to open. Thus, Russia did not get what it was supposed to get. It also weakened the ties between Russia and its allies, France and England.
This helped lead to WWI in part because it made it so that France and England pretty much had to back Russia in anything it did in the future. They'd backed out on helping their ally once and really couldn't do it again. This would be a big deal in 1914 when France and England entered the war in part because of their alliance with Russia.