Its right up there with:
decision to succeed from the Union by the South
Treaty of Versailles which in turn forced Germany into a corner and spawned WW2
leaving Pearl Harbor undefended when the indicators all pointed to a war was coming
Getting involved in Vietnam to start with
That is probably the top 5 and you could argue any of the others I mentioned being bigger mistakes.
if You French Fry when you should Pizza you are going to have a bad time