Final answer:
Russia promised to protect Serbia due to the Slavic connection between Russians and Serbs, as well as their shared rivalry with Austria. This eventually contributed to the outbreak of World War I.
Step-by-step explanation:
Russia promised to protect the Serbians due to several reasons. Firstly, there was a strong Slavic connection between Russians and Serbs, as they spoke related languages and both belonged to the same branch of Christianity. Additionally, Russia saw Serbia as a potential ally against its rival Austria. Serbian nationalists believed that with Russian support, they could create a crisis in Bosnia and ultimately seize it. However, their assumption that Austria would not risk war turned out to be wrong, leading to a series of events that ultimately escalated into World War I.