score:7
An invasion by US armed forces would have worked, but would also have been deeply illegal, since it would have been a war of aggression. It would also likely have precipitated a war with the Soviet Union, and Cuba wasn't worth that risk.
The plan seems to have started as one for a counter-revolution within Cuba with outside support, and been expanded into a plan for overt military action without that being properly thought through. The forces that landed, composed of Cuban exiles, were too small for the job and didn't have enough supplies or support. They also assumed they would have support from the population, but actually, most of them were pro-Castro.
Wikipedia's summary of the CIA internal report on the fiasco covers the matter reasonably well.