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.
The sleeper effect refers to a situation when the effect of messages become more stronger as time passes.
It is the situation in which a message presented by a high credibility source becomes less persuasive over time, while a message from a low credibility source becomes more persuasive over time
