In 1908 the status of Bosnia was somewhat uncertain, administered by Austria-Hungary but formally still part of the Ottoman Empire. Inhabited by slavs, Bosnia was part of the slav nationalist dream of a South Slav Kingdom (Yugoslavia).
The so-called Pan-Slav movement wanted all slav people to unite, and the most aggressive proponent of this movement was Serbia. The Pan-Slav movement was sponsored by Russia; the Great Power which also was a slav country. But St. Petersburg did this also for strategic reasons: if Serbia could grow stronger in the Balkans, possibly at the expense of Turkey, Russia might finally win free access and control of the Straits of Constantinople (also known as the Straits of Istanbul, or the Dardanelles).
The foreign ministers of Russia and Austria-Hungary, Alexander Izvolsky respectively Alois Lexa von Aehrenthal, negotiated in secrecy an agreement: Russia would accept an Austrian annexation of Bosnia and Austria would support Russia’s desire to get free access through the Dardanelles. The would be done at an international conference at a later date (not set at the time).
However, before Great Britain and France gave their approval for such a conference, the Austrians went ahead and proclaimed their annexation of Bosnia. This caused an outrage in St. Petersburg, in Belgrade and in other slav quarters. Meanwhile it seemed unlikely that London or Paris would accept a conference to facilitate Russia’s access to the Mediterranean…