You gain morale rank by killing enemies and unlocking new battle flags. If you die your rank will go down to the level that it was at the previous flags limit. For example if you are morale 10 and die but the last flag was 8 you will go back to 8. This will most likely be unlocked naturally but if you were like me and wanted to do it asap just find any battle flag with enemies near by. Kill them and go back to the flag post and interact with it to reset the enemies and rinse and repeat until level 20.