As most people know, the spark that led to WW1 was Austrian-Hungarian monarchy's invasion of Serbia. This is usually portrayed as a bloodthirsty and warmongering Austria-Hungary trying to dominate Serbia. However, this video gives a different perspective:
The summary is that the Black Hand in Serbia was used as a base of operations for carrying out terrorism in the Austria-Hungary. They couldn't really do much about it because they had no sovereignty over Serbia, so they could just carry out their terrorism and go back under the safety of Sebria. While there is no direct evidence the Serbian government was directly involved in Black Hand terrorism, it certainly appears that they were covering for them. The best case scenario seems to be that they were just plain incompetent in their handling of the situation, and that's being generous. The ultimatum given to Serbia is usually portrayed as completely unreasonable and just an excuse for war. But looking at the context, it appears to be less unreasonable than usually assumed.
I didn’t watch the video, so maybe this is mentioned, but Austria-Hungary issued an ultimatum to Serbia in the aftermath of the assassination of Franz Ferdinand that consisted of ten demands. Nine of these demands were submitted to. The one it did not submit to — although it agreed to submit the matter to arbitration — required Serbia to allow Habsburg authorities to conduct their own investigation on Serbian soil, which many considered to be a compromise of Serbia’s sovereignty.
Notably, Serbia agreed to crack down on the Black Hand’s parent organization (called People’s Defense), so I don’t think it’s fair to say that the Serbian government was covering for them. Further, relations between People’s Defense and the Serbian government had soured by 1914. Serbia had no reason to protect organizations endangering its own security.