Perform a perfect den defense without using the cannon.
You can get this most easily on your very first den defense, which is basically a tutorial. It is during one of the main memory sequences, Sequence 2 Memory 6, so it's impossible to miss and you can go back and repeat it any time. You need to perform a perfect den defense, which basically means no enemy reaches their objective and causes damage to your den. You cannot at any point use your cannon so just forget the button exists for 5 minutes. Follow the instructions given to begin with, placing a leader and some crossbow troops. From then on you should mostly concentrate on rifle troops and creating at least a few strong barricades in order to survive the last wave, which is a battering ram. You can also help out by moving the cursor to enemies and using your hidden gun. Also, you can loot some dead enemies by pointing at their bodies and using . This mostly only gets you money but sometimes some extra moral as well. The battering ram is the only real hard bit about this so just make sure you are set up properly for the final wave and you should be fine. Here is a video for the mission to help:
I didn't get this on the sequence 2 mission. I just played the game and defended the den's (mostly failing because of the cannon) but suddenly instead of the cannons/any other war-machine, stalkers started to arrive. So yeah quite easy after that.
If anyone's having trouble with this: play the tutorial level (sequence 2 level 6) and when it tells you to build a barrier just wait. Your morale will just keep going up until you build a barrier so you can wait until you feel you have enough and build an armada right away.
Looks like EndlessCola beat me to the punch here as far as getting a crazy amount of moral for the sequence 2 / level 6 den defense.
Just thought I'd also mention you can build your moral after you put an assassin leader on the second roof and right after you place your first firing assassin. Don't place your second firing assassin, just wait it out until you have enough moral that you're comfortable with.