The subtropical jet streams formate from cool and warm air masses that meet near the equator (close to the 30* Latitude).
The main cause was the assassination of Franz Ferdinand who was the archduke of Austria. He was assassinated by Gavriollo Princip who was Austrian - Hungarian. Those two countries were constantly in conflict because Hungary wanted to separate and once those countries went into war over the assassination their allies had to join being the rest of Europe. <span />
