Modern history
However, the crisis did not exist in a void; it came after a long series of diplomatic clashes between the Great Powers over European and colonial issues in the decade prior to 1914 which had left tensions high. The diplomatic clashes can be traced to changes in the balance of power in Europe since 1870. An example is the Baghdad Railway which was planned to connect the Ottoman Empire cities of Konya and Baghdad with a line through modern-day Turkey, Syria and Iraq. The railway became a source of international disputes during the years immediately preceding World War I. Although it has been argued that they were resolved in 1914 before the war began, it has also been argued that the railroad was a cause of the First World War. Fundamentally the war was sparked by tensions over territory in the Balkans. Austria-Hungary competed with Serbia and Russia for territory and influence in the region and they pulled the rest of the great powers into the conflict through their various alliances and treaties. The Balkan Wars were two wars in South-eastern Europe in 1912–1913 in the course of which the Balkan League (Bulgaria, Montenegro, Greece, and Serbia) first captured Ottoman-held remaining part of Thessaly, Macedonia, Epirus, Albania and most of Thrace and then fell out over the division of the spoils, with incorporation of Romania this time.

Q: What fundamental cause started the war in 1914?
tensions over territory in the Balkans.