On 28 June 1914, Gavrilo Princip, a Serbian nationalist, assassinated the Austrian heir to the throne, Archduke Franz Ferdinand and his wife Sophie in the Bosnian capital Sarajevo. People call this event the thing that sparked WW1.
I think the increase in military size was one of the causes of WW1. Other countries could've seen this as a threat to their country so they would increase military size and threaten the other countries.
Alliances was another cause of WW1 because other countries could've seen two other countries helping each other out as a threat and the country would have to make allies with other countries to defend itself.
Imperialism is another main factor that caused WW1 since thats how each country got their resources for war and industrialization which eventually led to more weapons of war.
Nationalism help start WW1 because people would glorify their country wich would make their leader spend more time and resources on the military and that made other countries feel threatened
Russo-Japanese rivalry over Manchuria and Korea reached its height with the Russo-Japanese War. The outcome of the war against the Japanese was a major blow for the Russians who lost almost entire Baltic and Pacific fleet. The defeat also provoked a serious political crisis that led to the Russian Revolution of 1905.