How did WWI start?How did WWI start: who were the main "actors". What were the different sides fighting for?
The main actors at the war's outbreak were enmeshed in the two major alliance systems of the early twentieth century. The Triple Alliance consisted of Germany, Austria-Hungary, and Italy; and the Triple Entente was made up of Great Britain, France, and Russia. Many of the nations also had alliance obligations to smaller nations, as Russia did with Serbia and England did, more or less, with Belgium. When the war broke out, of course, Italy did not join Austria-Hungary and Germany, but the Ottoman Empire, the other major player in the war, did. By the end of the war, the United States and many other nations around the world had joined the Allied fight against Germany, Austria-Hungary, the Ottoman Empire and Bulgaria, who rounded out the so-called Central Powers.
WWI started with the ultimatum made by Austria-Hungary to Serbia in response to the assassination of Archduke Franz Ferdinand. It then spread because so many of the "main actors" were in alliances with one another. For example, Russia was allied with Serbia and France and England were allied with Russia. Germany was allied with Austria-Hungary. Those alliances truly got the war started in a major way.
Don't underestimate the role that the growing sense of nationalism that was sweeping across Europe at the time played in the Great War. I agree with all the facts above, but in many ways most of the countries in Europe were looking for a way to assert their dominance and prove that they were ready to stand as the leader of the continent.
The conversion of the British Navy under Churchill to oil from coal meant a high risk strategy as England had abundant coal but no then-known oil. It secured a major concession from the Shah of Persia in the early 1900’s. The Baghdad rail link was increasingly seen in London as a threat to precisely this oil security. The British response to the growing German disruption of the European balance of power after the 1890’s was to carefully craft a series of public and secret alliances with France and with Russia—former rivals—to encircle Germany. As well, she deployed a series of less public intrigues to disrupt the Balkans and encourage a revolt against the Ottoman Sultan via the Young Turks that severely weakened the prospects for the German Drang nach Osten. The dynamic of the rise of German assertiveness, including in addition to the Baghdad rail, the decision in 1900 to build a modern navy over two decades that could rival England’s, set the stage for the outbreak of a war in August 1914 whose real significance was a colossal and tragic struggle for who would succeed the ebbing power of the British Empire. The resolution of that epic struggle was to take a second world war and another quarter century before the victor was undeniably established. The role of oil in the events leading to war in 1914 is too little appreciated...