Although many might say that World War I was caused by the assassination of Archduke Franz Ferdinand of Austria, his murder was only the spark that started the flame. There were four main factors that fed the fire of WWI, the biggest cause being alliances, followed by militarism, nationalism, and imperialism in that order.  

The main cause of WWI was the alliances throughout Europe. If these alliances hadn't been in place, it would have only been a small war between Austria-Hungary and Serbia. When Germany pledged their support for Austria-Hungary, they triggered a domino effect which caused Russia, France, Belgium, and Great Britain to become involved, forming the two sides, the Allies and the Central Powers.

Militarism, the building up of one's government, was also a significant cause of World War I.

At the beginning of the war, many countries built up their military, just in case they were drawn into the war, or because everyone around them was doing it. This was a main cause of the war, because everyone was prepared to fight, and this made conflict more likely to occur. 

Imperialism is when a country tries to further its power by gaining control over other territories. This was a main cause in WWI, because larger countries were competing to get colonies, and this may have caused tension between certain countries. 

Nationalism is an extreme form of patriotism, where people usually feel that they are superior to other countries. This was another main cause in World War I because many were sure that they could easily win the war, because they saw themselves as being far better than other countries, including in military-wise. They also wanted other countries to see them as superior, and wanted to gain as much territory as possible, and winning the war would gain them territory.

The majority of battles in WWI were fought in trenches. Living and fighting in these trenches was not pretty. The trenches were cold, wet, and rat-infested. Dead bodies would often be lying in the trenches or above them, in no-man's land. Diseases were very common in the trenches, between the bodies, rats, and small space.

The typical trench system had 3 or 4 trenches which were parallel to each other. These were the front line, support, and reserve trenches. The trenches were dug in a zig-zag pattern to give the soldiers niches to hide in. The depth of the trenches varied depending on the height of the men. Between the front line and no-mans land, there was typically a complicated barbed-wire fence to keep out enemies.

The war quickly became an arms race, with each side desperate to get the upper hand. While Germany was first to have machine guns, Great Britain was the first to have tanks, and it didn't take long for everyone else to get their hands on machine guns.