Can someone explain to me what was gained out of this invasion?
Well it screwed Fatah didn't it , that must have been the intention so it counts as a gain for the Israeli government doesn't it .

Hore those cluster munitions your post refers to is the bombardment of the bulldozed areas next to the prison fence to clear any possible landmines so it doesn't count on the list of war crimes commited during this pointless little middle eastern episode .