You are a small time emperor facing a huge empire comprised of a dozen de jure kingdoms. It's tough to win a war against such a gigantic enemy. And recently I have perfected a strategy for this situation.
First, you need to be an emperor yourself.
Then you must have full women's right and ability to make concubines, as well as ability raid. A reformed pagan with Equality would be ideal.
Now, very much needed, even though not absolute must, are Bloodthirsty Gods and a warrior lodge rank 2.
-------------
And here is how it work.
First you send your raiders to enemy's duke vassals' capitals. But make sure that the duke has a child, and the child's mother is in that capital. Most of dukes will have a wife and a young child from that wife.
When your raiders raid the capital holding, there is some chance of capturing the mother. Warrior lodge rank 2 and leading the siege personally increases the chance dramatically, around 80~90%.
Take the mother as you concubine and land her to a county with no vassal. If you captured all her children as well, then give her a kingdom title too. If not, just gift 15 gold and you can imprison her for 100%.
Execute or Sacrifice (Bloodthirsty Gods) her.
Her child becomes your vassal. If he wasn't already imprisoned. Now give him gold gift and a kingdom title. Then imprison and sacrifice him / her too.
The duke is now your vassal king. If your army is strong, he won't refuse when you revoke his kingdom title that you gave to his wife or son.
After trying all border / coastal dukedoms. Start a holy war to the enemy emperor, so that your army can revisit those dukedom capitals where you failed to capture the mother of the duke children.
When you have re-stormed all the target dukedom capitals, change your religion to invalidate the war you started.
Send your raiders to the new border dukedoms.
Repeat.
First, you need to be an emperor yourself.
Then you must have full women's right and ability to make concubines, as well as ability raid. A reformed pagan with Equality would be ideal.
Now, very much needed, even though not absolute must, are Bloodthirsty Gods and a warrior lodge rank 2.
-------------
And here is how it work.
First you send your raiders to enemy's duke vassals' capitals. But make sure that the duke has a child, and the child's mother is in that capital. Most of dukes will have a wife and a young child from that wife.
When your raiders raid the capital holding, there is some chance of capturing the mother. Warrior lodge rank 2 and leading the siege personally increases the chance dramatically, around 80~90%.
Take the mother as you concubine and land her to a county with no vassal. If you captured all her children as well, then give her a kingdom title too. If not, just gift 15 gold and you can imprison her for 100%.
Execute or Sacrifice (Bloodthirsty Gods) her.
Her child becomes your vassal. If he wasn't already imprisoned. Now give him gold gift and a kingdom title. Then imprison and sacrifice him / her too.
The duke is now your vassal king. If your army is strong, he won't refuse when you revoke his kingdom title that you gave to his wife or son.
After trying all border / coastal dukedoms. Start a holy war to the enemy emperor, so that your army can revisit those dukedom capitals where you failed to capture the mother of the duke children.
When you have re-stormed all the target dukedom capitals, change your religion to invalidate the war you started.
Send your raiders to the new border dukedoms.
Repeat.