If all your soldiers are wiped out in a fight where they are indeed supposed to die, you can use the "resupply" option to replenish them with new ones. You can also pay money or join forces with other players to resupply - this means the game will take the necessary measures and give you additional troops even if it's not your turn (which is called "force import"). Resupplying takes 1 hour and costs money (indicated by a clock icon) - it can also be done instantly by spending gems (indicated by a gem icon).