There were few valid reasons for the start of WW1. The reasons I am about to explain to you are IMPERIALISM, MILITARISM and ALLIANCES. WW1 started because the greed of other countries.
Imperialism started to be a big factor in 1848 because a new, very big and very strong country called Germany was made. It was made up of Prussia, Oldenburg, Schaunburg-Lippe, Mecklenburg-sterhitz, Hessen, Hohenzollern and Waldeck. A few days after they became Germany, they attacked France and conquered some of their land (Alsace and Lorraine). This is just one example showing that Germany want to become stronger and bigger than they were before. The Russian empire controlled Poland which showed they were making a advance to get a port at sea so the could have a use for their ships that they have built (dreadnoughts). Also the Ottoman Empire was decreasing fast so they would also want to gain some land back.
The arms race and the naval race were also a great long term cause of WW1. As one country increased its armies, so all the others felt obliged to increase their armed forces to keep the balance of power. The arms race was clearly being won by Germany with 8.5 million men in 7 years, that is 4.1 million more than Russia, who is in second place. On the whole though, the triple entente (Russia, France and Britain) had more arms than the triple alliance (Italy, Germany and Austro-Hungary). The naval race, although, was won clearly by Britain with 29 dreadnoughts and 9 warships while Germany had 17 dreadnoughts and 7 warships. Great Britain felt threatened by Germany for their sudden manufacturing of dreadnoughts because they could now takeover colonies that England have, so England made a alliance with Japan in 1904 so the pacific sea was taken care off.
Alliances were one of the main causes of WW1. There were two main alliances one was the triple Entente ( England, France and Russia) and the other was the triple alliance ( Germany, Austro-Hungary and Italy). When Franz Ferdinand, the archduke of Austro-Hungary was assassinated by the BLACK HAND organization they wanted to attack Serbia, but Russia had a alliance with
Serbia and vowed to defend them if Austro-Hungary made a move. If they did make a move though, Germany was in an alliance with Austro-Hungary, and also vowed to defend them if Russia attacked. If Germany attacked Russia however, France would defend Russia because they were in an alliance. This was the main reason that WW1 started, turning from a argument between two countries and turning into a war within the whole of Europe, including the USA.