In 1914, the European continent was divided between two rival sets of nations. One set was dominated by Germany and Austria-Hungary, the other by France, Britain and Russia. The countries had secret alliances, internal politics and the desire to grow empires.
This created a very tense situation in Europe. It also made for an extremely dangerous war.
The main reason for this was that the two power houses of the world were competing to build large worldwide empires. This was called imperialism.
During this time the German and French armies had been increasing in size. They had also started to introduce battleships that were much more powerful than the old sailing ships that they had.
This caused a very big arms race between the two powers. They had to have the best weapons and ships so that they could attack each other.
When the armies from both sides came together they had to be able to fight each other fast and they had to move very quickly. This made the war very hard to stop.
By the end of August 1914 the two countries were at war. The main reason was because Germany wanted to invade Belgium but the other country was also trying to stop them.
The other country that was trying to stop the Germany invasion of Belgium was the United Kingdom. They were also worried about the balance of power in Europe and how they would look in the future if the war went too far. This was why they decided to join the war.