What are the pros and cons of the alliance system ww1?

They took it too seriously. However, there are other factors that are independent of the alliance system that also contributed to the mess. It's been close to 100 years since the Napoleon was defeated, there was relative peace for far too long, and those European countries forgot that they were turning their guns on each other, not some backward third world country.

There weren't any pros. The cons however, was a list a mile long. Everything from limited economic relationships and lack of communication, to agreements that needed to be updated regularly to accommodate changes in the political climate.

World War One was a case study of just how fast everything can go horribly wrong.