At 1878 there was a war between Russian empire and Ottoman Empire , after the war a part of the Ottoman empire was considered Austria-Hungaries but was officialy declared as ottoman empires

As you can see here the balkan parts were considered as the part of ottoman empire but then after a few years Austria-Hungary invaded that part of ottoman empire. Ottoman empire let go that part of land cause Russia agreeded on that invasion , they also invaded Serbia. here is a historic cartoon on it :

"As you can see the person wearing a red cap is the Ottoman Sultan aka Prince who cants do anything while the Austria-Hungary is invading on them"

While Russia aceepted this invasion , why ? Russia had a connection with the another part of sea from the black coast :

It was known as the Strait of bosphorus

After Serbia wwas invaded the slavic countries didnt like that and then at one of the south slavic the prince visited and then

At 28th June 1914 at Austria-Hungary the royal prince  Archduke Franz Ferdinand was killed well it was before the year ww1 started when Serbia and Austria-Hungary were in war called the Balkan wars were Serbia needed some land from Austria-Hungary , after the war Serbian nationalists wanted to free the south slavs from Austria-Hungary , they wanted  to unify them as a nation. There was an extreme nationalist among them called Gavrilo Princip , the prince visited Sarajevo, a nationalist place in Austria-Hungary , when he was going there was a Serbian nationalist who threw a bomb at him which missed and hit the guards. He had a scheduled meeting with the people who wanted to show him the place but he left with his wife and driver to the hospital to see the guards. The driver did not know the way , he made a mistake and took a wrong turn. Gavrilo Princip was having a drink , he could not believe his luck when he saw the prince and his wife , he came and shot them and the Royal prince died. This made Austria-Hungary believe that the prince was killed by a Serbian nationalist. Austria-Hungary asks Germany and declares war on Serbia. They thought it was a Serbian man who shot the prince, thus they declared war on Serbia .

Serbia had alliance with Russia,Britain and France , from there World War one starts with the assassination. Just a recap of the biggest cause of ww1 

Mistake Done

After the bomb killed the guards , the king ordered the vehicle to go to the hospital, the serbian nationalist thought they lost the chance to kill the prince but a young teeneger serbian nationalist saw the prince take a wrong root , he came out of a cafe and shot him and his wife. This caused the ww1 as Austria-Hungary invaded on Serbia