You're right that a single act like this should not be a cause of a major war. In 1914 Europe was a place of alliances, formal and otherwise, Britain and France, France and Russia, Germany and Austria-Hungary, and Russia had close bonds with their Slav cousins in the Balkans, who were ruled by Austria. Added to this were imperial ambitions, primarily of Germany, who had been flexing their muscles since unification under Bismarck. Gernany believed that it should enjoy similar imperial spolis as Britain but was prevented by the strength of its navy. Germany embarked on a programme of naval expansion around the turn of the 19/20th century, causing friction with Britain. Friction with France was a permanent feature. Russia wanted access to warm water, and had been eying the Balkans for years. With these powerful alliances and ambitions, it was only going to take a spark to ignite it and this came at Sarajevo. Franz Ferdinand was heir to the Austrian throne, and was paying a visit to one of the empire's outposts. The Serbs wanted to overthrow Austria and achieve independence. Prinzip succeeded in killing th Archduke and his wife. Austria came down hard on Serbia: Russia came in to support them. Austria pushed things further than they should have, knowing Germany were in their corner. Germany should have reined them in but didn't. Russia declared war on Austria, Germany on Russia, France on Germany. At first, Britain stayed neutral, although links with France would have brought them in eventually. Britain got involved because of the Schlieffen Plan, designed to outflank France's Maginot Line by invading Belgium. Britain had guaranteed Belgian sovereignty by the Treaty of London (1820), but Germany did not think they would honour what they described as a "piece of paper". Within 12 days of the assassination most of Europe was involved in the most disastrous war the world had ever seen,and which in terms of military casualties is still the most expensive ever.