Apis' chief aide, Major Tankosic, recruited three young Bosnian Serbs from the seamy coffee shops of Belgrade. Gavrilo Princip, Nedjilko Cabrinovic, and Trifko Grabez were trained and supplied from a base in Serbia. Princip was ultimately successful in carrying out their mission. The assassination became the spark that ignited a world war. In March 1917, Apis was arrested in a government crackdown on the Black Hand.
Several theories exist for why.
One, is that Prime Minister Pasic and the Prince Regent were preparing to negotiate a separate peace with Austria and that they feared Black Hand reprisals. Another theory was that Pasic wanted to eliminate Apis and the others because they could expose government involvement in the Sarajevo murders. Yet another theory is that Apis was actively subverting the government. For whatever reason, Apis and many others received a rigged trial before a military tribunal. Apis and three others were sentenced to death for treason.