They did, this is what started the First World War.
Russia started mobilizing its army to get the Hapsburg state to back down in the Austro-Serbian crisis. Actually many Slavs were within the Austro-Hungarian Empire while other people were already within the Russian Empire.
On March 3, 1918, the treaty of Brest-Litovsk was signed. This was between the central powers i.e. Bulgaria, Austria-Hungary, German Empire, and the Ottoman Empire and new Bolshevik government of Russia.
About 8,500,000 soldiers died in this war. Military deaths were 5 million, and civilian deaths were 3 million.
World War 1 is also known as the Great War which started in 1914 and ended in 1918. A lot of people died in the Great War. A total of 20,000,000 people from all the nations died, including civilians and soldiers.
Machine guns, aircraft ships, rifles, artillery, submarines, and poison gas were used.
The Americans used a shotgun during the war. Germany used Zeppelins which was a type of aircraft, submarines U-boats, along with tanks. Mustard gas along with many other chemicals was also used in World War 1. This gas caused blinding and death by choking.
Germans were more ready for war, and they had a better plan.
The Germans had been wanting for a war for some time. The Germans implemented the Schlieffen Plan, in which the Germans left a small force in the south to resist the anticipated French invasion and sent a large force to crush them.
It meant Militarism, Allies, Imperialism, and Nationalism.
One way to remember the four leading causes of world war 1 is to remember the acronym M-A-I-N. Where M stands for Militarism, A stands for Alliances, I stands for Imperialism, and N stands for Nationalism. These are the major causes of war.
In 1914, World War I started, and it lasted for 4 years. During the war, Germany, Ottoman Empire, Austria-Hungary, and Bulgaria fought against Russia, Great Britain, Japan, Romania, Italy, USA, and France.
The plan of Schlieffen resulted in a stalemate. This was so because the German high command did not sufficiently commit to that plan.
The plan called for a very token force to be left to defend Germany against a possible French attack in the south. In the meantime, a right-wing was made to come around from the north and crush the French. But the German high command made the left wing too strong and right-wing relatively weak. Thus the right one was not able to break the French and result in a stalemate.