After the fall of the Ottoman-Turkish Empire, a group of new nations emerged on the Balkan Peninsula. During the definition of its territorial limits, serious diplomatic disagreements occurred between the States formed there. Supported by the Austro-Hungarian Empire, Bulgaria – in 1913 – ended up coming into conflict with Serbia, Romania, Greece and Montenegro. At the same time, the Slav peoples of Bosnia and Herzegovina, supported by Serbia, tried to end the Austro-Hungarian domination in the region.
In an attempt to circumvent the conflict, the Austro-Hungarian Empire decided to elevate Bosnia and Herzegovina to the status of an integral part of its state. That's when Archduke Franz Ferdinand, prince of the Austro-Hungarian Empire, went to Sarajevo, capital of Bosnia, to announce the political elevation of these territories. However, Francisco Ferdinando ended up the victim of an assassination plotted by the Serbian nationalist faction Black Hand.
In response, the Austro-Hungarian Empire demanded that the Serbian government harshly punish the perpetrators of the crime and repress all the country's radical factions. Without obtaining satisfactory answers, the Austro-Hungarian Empire decided, with the support of Germany, to declare war on Serbia. Russia, opposed to the Austrian occupation, granted support to the Serbs. Soon after, Germany declared war on Russia and, later, on France.
Italy and England did not show a clear political definition regarding the conflicts. However, the strength of the diplomatic agreements signed and the economic interests forced both countries to get involved in the started war. The generalization of the conflict had the subsequent adoption of other nations, including Brazil. In general, the conflict divided the European powers into two blocs: Triple Entente, formed by France, England and Russia; and the Triple Alliance, formed by Germany, Italy and the Austro-Hungarian Empire.
20th century - wars - Brazil School
Source: Brazil School - https://brasilescola.uol.com.br/guerras/oinicio-primeira-guerra.htm