Answer:
Large Standing Armies and Treaties.
Step-by-step explanation:
Countries built large standing armies and signed protective treaties with each other because they feared each other's armies. This turned Europe into a powder keg and all that was needed was a spark to set it off. This occurred when a Serbian nationalist assassinated the archduke of Austria-Hungary. When Austria declared war on Serbia, Serbia's allies came to its rescue. Then Austria's allies came to support Austria. Overall, this led to almost every country in Europe joining the war. Not to mention, the large standing armies meant that the loss of life was miles higher than any other previous war.