They had both seen Rico (and rebel factions) defeat their militaries, so evacuating the area of certain doom made sense.
And they both had a plan:
Mendoza actually had 2 plans: Plan A: Get flown to some other country by his presidential cargo plane. Plan B: Parachute to a submarine that Rico lacked the means to damage and get transported to another country. As an actual military man, he would have known the value of a strategic retreat (or of retirement)
Espinosa was all about personal gain and massive ego. He had nothing left to fight for at Solis. At least not with what he had left (like maybe a handful of body guards). The plan was to go sell the weaponized weathermachine to the u.s. for "trillions". After that, he would still have controlled many Black Hand units who were left in other countries.
In either case, there's a famous quote: "Only a fool fights a battle that he knows he can not win." Do you know who said that? Genghis Khan said it when his home village was attacked and he was forced to become a refuge. He went on to build the most powerful empire ever to have existed. By the time he died of age, his armies had conquered half of everything he knew existed.
