Who do you think really started this war?
It's not so much who started WW1. It's who could have stopped it.
Each of the five European powers that went to war in July of 1914 could have stopped things from escalating to the level they did. But the network of alliances, intense nationalism, arms race, and fears among military commanders that their nation would fall behind in preparing for war ensured that conflict would happen.
Serbia had agreed to almost all of Austria-Hungary's ultimatum. What if cooler heads in Vienna had prevailed, and Austria-Hungary had accepted this gesture? What if Russia had not moved wrecklessly toward mobilizing its enormous army, throwing the Central Powers into panic, not once but twice, and allowed Austria-Hungary to handle Serbia on its own? What if Germany had not given Austria-Hungary a "blank check" saying it would support anything and everything Vienna decided to do toward Serbia, and any resulting consequences? What if France's military staff had not goaded the Russians toward mobilization, motivated partly by France's eagerness to regain control of Alsace-Lorraine?
And what about Britain? London made a serious effort to diffuse the situation, but
all the while never made it clear what it's actions would be. If Britain had made
it clear they would enter the war on the side of France and Russia, it's unlikely
Germany would have gone as far as they did in supporting Austria-Hungary. On the other hand, if Britain had made clear they would not enter the war, it's unlikely Russia and France would have felt so confident in mobilizing their armies.
One thing you won't often find in the history books is that the powers of Europe almost went to war four times in the years leading up to WW1. Each time, conflict was avoided, not because of a desire on all sides to find true peace, but because one side was forced to back down. This resulted in humiliation and feelings of resentment, so that by 1914, nobody was willing to back down any longer.
World War I is really tragic. Europe was at the height of its prosperity and times were good. The continent decided to abandon all that for a war that everyone thought would be short, but which turned out to be the most horrible in its history, arguably.
A lot of countries in my opinion.