What units kill and defend from what

You use strike to:
- Mass a defense [Assault]
- Farm/raid [Farm/Raid]
- Kill assassins [Hunt Assassins]
- Mass the enemy strike [Destruction]
- Kill enemy miners and workers [Invasion]

All attacks except the Assault type will cause little or no damage to the enemy defense

Defense protects your assets and your income, helps only against strike.

Covert is used to:
- Recon enemy stats
- See basic stats (like army size and kuwal out)
- Sabotage
- Recon enemy trade logs (not yet coded but it'll come)
You defend from sabotage with your own covert power
5% agents lost when success, 75% agents lost when failed

Assassination is used to kill units of all types:
- Offense
- Defense
- Spies
- Assassins (it's possible but unless there's a big difference in assassination tech and skills you'll rarely use it due to its costly nature)
You defend from assassination with your own assassination power
5% agents lost when success, 75% agents lost when failed
Assassinating someone who has a higher assassin SKILL (Not Tech or Action BUT SKILL) than you will cause you to send more agents to kill the same amount of enemy units than you would need to send if you had higher assassin skill than the target

In the case of protecting from sabotage with spies and all forms of assassination, increasing your realm alert will increase the amount of enemy agents your opponent has to send in order to succeed.
Turning on realm alert will prevent you from being able to order any missions to your units.