In 19th centaury there was a war between Russian Empire and Ottoman empire for a part of Istanbul.
When WW1 happened the map of Europe:
In the 20th centaury Austro-Hungary Empire Declared a war against Bosnia country(It is a country right now, but it was a part of ottoman empire) for imperialism. Ottoman empire was already too weak so, we had to let them capture. But the people living there were not very happy about it. They were part of Ottoman empire for 400 years but instead of getting freedom, they are being captured by another country. Austro-Hungarian Empire go to Russian Empire that if they invade Bosnia then they would not make any objection. In return if Russian Empire Invades Ottoman Empire they would not help Ottoman Empire. Russian empire agreed to this deal.
On the other hand, the southern countries of Europe were supporting Bosnia. Because the people of Southern countries wanted a merged countries of Southern countries. They called it Yugoslavia. Serbia was one of the countries.
Once the price of Austro-Hungary Empire was on a visit to Serbia with his wife. While on the visit a group of young young people throwed a bomb in his car. But it hit the car and injured 20 officers behind him. Then the price was on his way to the hospital to see the injured people. But unfortunately they took a wrong turn and another 19 years old boy shot him. He was assassinated with his wife. In court the man said that he was a Yugoslav nationalist, his goal is to re-create Yugoslavia.
After that, Austro-Hungary Empire was so Angry about it. They gave a list of some impossible demands to Serbia. When Serbia Declined it, Austro-Hungary Empire Declared a war against Serbia.
On the other side, there were many Sothern countries native people living in Russia and they were protesting to support Serbia. Russia had to listen to their people.
But Austro-Hungary Empire already guessed it and said to German Empire that if Russia come to the war will Germany help? Germany agreed because they were very old friends.
Then Russian Empire got to France and said Germany and Austro-Hangry Empire are going against Serbia so, if Germany comes to the war will you help me? France also said yes.
France and British Empire were very close friends so, British Empire also joined the war.
Portugal had good relation with British Empire and bad relation with German Empire so, Portugal also joined the war.
As Ottoman Empire was a enemy of Russian Empire, so Ottoman Empire joined with Austro-Hungary empire.
Italy had a good relations with Austro-Hungary and German Empire. But Serbia Team promised Italy that they will some land from Austro-Hungary Empire so Italy switched side.
Bulgaria wanted to be bigger and Austro-Hungary team promised to give some land so they both invaded Serbia.
Greece joined Team Serbia.
Germany wrote a letter to Mexico to attack USA. But while it was going to Mexico, UK founded the letter and showed it to USA. Then USA joined team Serbia.
Team Serbia:
- Serbia
- Russian Empire
- British Empire
- Portugal
- France
- Italy
- Greece
- USA
Team Austro-Hungary Empire:
- Austro-Hungary Empire
- Ottoman Empire
- German Empire
- Bulgaria
A great war had begun. But both teams were so powerful so, there was not coming an answer of who is winning or who will win.
Russia broke down first then the other countries.
After 20 million deaths the war stopped.