The question of which country actually started WWI will probably never be answered, because no one can quite decide which nation is most at fault. That doesn’t mean there isn’t a strong contender. That’s right, we’re talking about Germany!
What evidence is there that Germany was responsible for the war, you ask? To start, Germany encouraged Austria in its aggression against Serbia, declared war on Russia and France, invaded Belgium and hoped to extend its lands in the east and to gain colonies from Britain and France. So, a lot.
In fact, there is much evidence to support the view that Germany was responsible for the outbreak of war in 1914. The argument works on the assumption that Germany encouraged Austrian aggression towards Serbia because it knew that their war plans were ready and they needed to make a first strike in order for the Schlieffen Plan to be effective. The argument next sees the German attack on France as a necessary part of the Schlieffen Plan which involved the invasion of Belgium. By this logic, it was the violation of Belgian neutrality that brought Britain into the war against Germany. Additionally, German rulers hoped that Germany would gain territory as a result of the war. So, they were a bit selfish really.
On the other hand, there’s also weight to the argument that Germany wasn’t responsible for the war. Confusing, I know! The argument for this is based on the assumption that the war would have happened anyway because of the alliance system. This side believes the alliance system made war much more likely whether or not Germany started it. The argument works on the assumption that Austria would probably have attacked Serbia with or without German encouragement, and that Britain was very concerned about the expansion of the German Navy in the years leading up to WWI. Going by this theory, it is possible the German invasion of Belgium was merely used as a pretext by Britain in order to enter war against Germany.
The annoying bit is that we’ll probably never know.