The last straw that caused that triggered the outbreak of WWI was the assassination of Archduke of Austria - Hungary Ferdinand and his wife on June 28, 1914 by a Serbian nationalist with ties to the secretive military group known as the Black Hand.
Due to this, one month later, Austria - Hungary declared war on Serbia. The situation was joined by other countries based on which alliance they were in. A group of alliances between major powers in Europe had been formed or were in the process of being formed by this time.
But it's politically charged. Another answer is that great Britain wanted to conquer the Turkic Islamic empire of the Ottomans in order to colonize Asia. Once this was accomplished, the oil companies moved into the middle East and forced the Arab nations into giving up sovereignty over their national resources. Another name for the war is the Second War of Ottoman Succession.
While both answers are true, they have propagandistic elements and you can slant your writing by de-emphasizing one or the other. It's not the case that the war was "caused" by the death of an obscure aristocrat. That statement there, that that was the cause, smacks of being an excuse. The causes are complicated by rising militarism and secret treaties. Because nations were forming alliances without telling each other, nobody could have foreseen that the murder of Ferdinand by some marginal terrorist (again, slant--how marginal were the Young Turks? Why did they exist?) was going to force Germany to march on Paris. Those: the rising nationalism, militarism, secret treaties, and desire for control over Asia--are all causes--which is why the assassination of Ferdinand is considered the last straw, not a cause in itself. It's considered a "proximate " cause.
We are told that it's the cause for the same reason we are not told about the Ottoman empire and the Seven Sisters oil cartel and T.E. Lawrence and Redline and Achnacarry and the San Remo treaty, none of that stuff. Ferdinand's death had so little to do with the causes--the secret treaties--that it's ridiculous.
The framers of the United States Constitution based our federal government on federalism. ... Federalism has evolved over the course of American history. Some important events have shaped the balance between the national and state governments so that federalism best suits the needs of the country at that time.
