Why was World War 1 not fought just in Serbia and Austria-Hungary?

Answer 1

Russia joined the war because it considered Serbia to be its "brother" as they were both Slavic. The Triple Entente had been formed which was an alliance between Russia, France, and the United Kingdom, which all these three countries had agreed to join in and help if any of them should be involved in a war, thus resulting in the First World War.

Answer 2

Because Austria-Hungary and Serbia were allied with other countries.

• Serbia was allied with Russia
• Austria-Hungary was allied with Germany and Italy
• Russia was allied with England and France
• England was also allied with Belgium

So the sequence of events goes as follows:

1. the Austrian heir is assassinated, and Serbs are believed to be involved.
2. after a bunch of diplomatic stuff, Austria declares war on Serbia about a month later.
3. Russia declares war on Austria
4. Germany declares war on Russia.
5. Germany knows that Russia and France are allies, so they also declare war on France
6. France declares war back on Germany
7. Germany's war plans involve going through Belgium to attack France. So Germany asks Belgium to let the German Army go through their land, and Belgium says no.
8. Germany invades Belgium anyway and then attacks France as planned.
9. England declares war on Germany.
10. The Ottoman Empire is given some ships by Germany, and those ships then attack Russia- so the Ottoman Empire was forced to join the war on the side of Germany (they were secretly allied so it would've probably happened anyway).

So all these other countries wound up getting involved. The fighting in France wound up being the focal point of the war in the west; the war was also fought in the border lands between Italy and Austria (Italy eventually broke their treaty with Austria and joined the side of England, France and Russia), in Russia, Africa, the Middle East, even Asia and the Pacific.