[Casus belli] How can I get a "Invasion" Casus belli?

[Asratius]

In my last game as Torrhen Stark, I saw Loran Lannister holding a Council with his vassal to discuss about a kingdom to invade. After doing this, he declared war over the Iron Isles to conquer the Riverlands region. He used a "Invasion" CB. How can I get this CB?

 

[Moarice]

Pick "Expand the realm through conquest" ambition and pick the right decision.

 

[Asratius]

Thank you. However I can't pick that ambition because I have the content trait