The thing I always remember, one of the few, in fact, I ever remember about WWI was a teacher in high school said "World War I started as the world's biggest family feud'.
That is, all the royalty in Europe were either siblings, cousins, nephews, etc. The Archduke, or heir to the Austrian throne was murdered by a Serbian (Yugoslavian) rebel, who wanted his country to be free of the rule of Austria-Hungary. The Russian Czar actually was backing independence for the Serbs. Russia was thought to be part of the conspiracy. Russia was allies with England (and the Czar and King were cousins) and France, which no longer had a king; but anyway the kings were against the other cousins, Kaiser Wilhelm of Germany and I forget the name of the Austrian Emperor. Despite what someone said, Austria and Germany did not go to war against each other, Germany and Austria were allied, called the Central Powers, and Britain, France, Russia and later the US were the Allies, like they were in WWI.
So at first, it was a lot of royal cousins sending their armies into battle. As time went on, Britain got more and more aid from America, and we eventually joined. That is complicated, I'd suggest you look it up. I believe, unlike WWII, Italy was on the side of the allies. There were some other countries involved, but US, after 1917, Britain and all it's colonies, France, Russia, until they dropped out due to the revolution; vs Germany and AUstria-Hungary.
One of the worst things about WWI was the use of mustard gas and other chemical weapons. ALso planes were used in warfare for the first time, as were tanks. SOme of it seems fairly primitive today, but they were very advanced for the time.