When Serbia decided that it was time for them to expand, Austria thought that they were becoming too powerful. Austria was looking for a way to destroy Serbia, while making sure that no one would go and help Serbia in the war. When Germany heard that Austria was looking for a way to attack Serbia, Germany promised that if Serbia did go to war, they would keep Russia, France, and Britain out of the war.
At the same time the Austrian emperor's nephew was visiting tourist sights close to Serbia. When Serbian terrorists heard about it, they went and killed the emperor's nephew. After this, Austria became angry at Serbia and they made a list of demands that the Serbians had to follow. The Serbians disagreed with one point so Austria went to war against Serbia. When Russia heard of Austria's attack on Serbia they announced war on Austria.
Germany having promised Austria that they would keep Russia out of the war, announced war on Russia. They also attacked France to make sure that they didn't join in and make the first move. When the Germans attacked France they wanted to attack them from behind, so they marched their troops through Belgium. When this happened the British Government was called to war, since Belgium was under protection by all of Europe including Britain.
When Britain joined the war, Canada, being a British colony was obligated to enter the war to defend Britain. So Canada joined the war and went off to the battle field.
