That's why relying on mercs (which is what both UAE and Saudi chose in Yemen) is a double edged sword. At some point the cash isn't enough of a motivator and they will just run away.
It's not a perfect book, but it illustrates a lot of military failures that haven't really been encountered much in the West in the last 80 years.
People need to keep in mind tactical versus strategic success. For Iran, the military operation was a complete tactical success, it's just that the strategic goals — assuming there were any to begin with, which seems to be an unanswered question — were not, and it was questionable whether military operations were a useful tool to achieve them at all.
“War is the continuation of policy with other means.” — Carl von Clausewitz
'What is one's stated policy / goal, and is war a useful tool for achieve it', is a question that needs to be asked by civilian leadership. Sometimes (often?) the answer is "no".
I don't think native Yemeni are motivated enough to fight for unification. That's why all the fighters are mercenaries hired by the UAE or Yemen
idealogical fighters fight to the death. & in almost every case besides the nazi's etc they always win. whether the ideology is right or wrong that's a conversation for a different day.