Simplified in the best way possible:
World War 1 started when Archduke Franz Ferdinand was shot and killed by a Serbian man. As of a result, Austria demanded territory (taking advantage of the fact that the killer was Serbian). Austria declared war on Serbia. Serbia was in an agreement with Russia which led to war. Germany declared war on Russia for declaring war on Austria. Germany knew that France was allied to Russia, so Wilhem II declared war on France (it was going to happen anyways.) Britain later declared war on Germany after they had declined an ultimatum. During the war, UU Boats attacked British boats, and tried to starve them. While attacking British boats, they had accidentally attacked an American steamboat. This had caused tension. Germany agreed not to do it again. A few years later, they had started attacking American boats, thinking that the Americans were supplying the British. Tension grew again, which led Germany to send a telegraph to Mexico asking them to attack the United States, promising them their old lands. Since Mexico was at a civil war at the time, their generals told their leader not to attack the United States, and side with them. Mexico did exactly that, and had informed the United States. The people's will to fight in the United States grew, as mobilization started. In 1917, the United States declared war on Germany and sent thousands of troops to France almost every week.
Serbia was to blame as a part of their military known as the black hand secret society had trained soldiers to kill Ferdinand and split Austria's provinces so Serbia can form Yugoslavia. The head of the black hand secret society was the chief of the Serbian military intelligence.
