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.
I believe he is drawing a parallel between a tone of pure joy and the most depressing despair.
