I lost two raids at a village so drop pod in some troops the third time and succeed.
was this a game mechanic or just the dice of the universe.

If random then : Maybe this could be a mechanic. maybe a quest is formed and you can choose to send a number of troops that have a chance of being injured or dying but improve the chance of a settlement to hold out.

OR if there is a player caravan at the location when the raid happens they provide a boost of some kind.

OR maybe sending weapons or medication or armour equivalent to some cash value would give a one point increase to local military temporarily.

OR maybe let the players base be a option for defence I don't know how the players base defence would be rated in comparison though.