These alliances essentially decided who was fighting who. Countries who were allies with each other ended up fighting together. For example, the spark that began WWI was the assassination of Archduke Franz Ferdinand of Austria; after his assassination, it was essentially as follows: England joined the war because Germany attacked Belgium trying to get to France because France was allies with Russia who was allies with Serbia who wanted land from Austria-Hungary, which is why Archduke Franz Ferdinand was killed. Then the US and other countries joined because of threats to their allies and threats from other countries and so on. So allegiances played a big part in not only where the war was being fought but who it was fought by. I hope this helped!
These alliances that took place before World War 1 in a way started the war. For an example, Germany was in an alliance with Hungary and when they declared war against Serbia, Germany had to fight with them, and so on, finally creating World War 1. Within weeks, Germany declared war with other countries that declared war with others, creating a "domino effect" which just means that most countries just started fighting with one another, either because they declared war with them or because their allies did.
Revolution means an overthrow of a government, usually through the use of force, such as a coup d'etat. It could also mean a scientific or artistic breakthrough.
