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".
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.
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