A Cassie is the answer I think
Honestly google could help better since the majority of us don't know the book as well as you
I believe the answer is: "Resistance to civil government (Civil Disobedience)"
The main idea of the civil disobedience is to advocate the people to not ever let the government overrule their conscience .
He pointed Mexican–American War as one of the example because he perceived the war as an act of injustice for the people of Mexico.
