History Discussion: "It sounded good on paper!" Moments in history (Pre-1900)

Ideas or strategies or plans sounds usually good on paper. In most cases in history, there are plans or strategies or ideas that work out for the best, that live on in the history books.

We're not talking about those moments. Instead, we're going to look at moments where ideas or plans or strategies, whether that be in architecture, battle strategies/tactics, political intrigue, etc, where the idea sounded like a good idea, but then either lead to utter disaster or backfired or had unintentional consequences in hindsight.

What is one moment in history, where reading the ideas or strategies or plans, etc from someone or some people that sounded like a good idea on paper, but instead just failed miserably?
 
Killing Julius Caesar to restore the traditional norms of the Roman Republic. They managed to do the first part, but it might have prevented them from accomplishing the second part.
 
Top