Answer:
Long Story short...
Step-by-step explanation:
BACKGROUND:
Before WW1 starts many of the European superpowers (Britain, France, Germany, Austria-Hungary and, Russia) all were building up their militaries because of new inventions like machine guns. When countries build up their militaries that causes tension between other countries which can lead to issues.
AUSTRIA-HUNGARY:
The Austrians are going to take a country called 'Bosnia-Herzegovina' and that's really gonna anger Serbia which lives near Austria. Many Serbian-Nationalist are going to protest and one in particular (Gavrilo Princip) is going to assassinate Archduck Franz Ferdinand (Heir to Austrian throne) and thats really gonna make Austria mad and their gonna send an Ultimatium or a list of demands and Serbia is gonna decline and Austria hungary declares war on Serbia.
RUSSIA:
Russia is upset about this declaration of war because like Serbia both countries are Slavic and Russia feels obliged to protect Serbia so Russia begins mobilizing their troops and gives support to Serbia. So than Russia sends their own Ultimatium to Austria which is denied and so than Russia declares war on Austria.
GERMANY:
When Russia starts mobilizing her troops, Germany is forced to do the same for protection, and with the alliance made between Austria-Hungary and Germany, Germany joins the war against Russia and Serbia, and because of Russia and French alliance Germany than declares war on France before France gets the chance to.
FRANCE:
With declaration of war France begins mobilizing her troops and ask for Britain to join (which they will after Germany invades Neutral-Belgium).
Pretty much the foundation for WW1 is built on Alliances, Militarism, and Nationalism as well Imperalism