.Who does Thoreau believe is to blame for the injustice? What does he believe people should do regarding the injustice in America?

The people of the US and their government

Explanation:

Thoreau argues that the United States' invasion of Mexico is immoral and that Americans who support the government with their person (as soldiers) or property (through taxes) are complicit in that injustice. He would further say that a person should go to jail rather than be responsible for that invasion.

In practice, the opponents to reform in Massachusetts are not the Southern politicians everyone blames for extreme conservativism. Rather, they are the people who passively tolerate the status quo: merchants and farmers in Massachusetts who are not willing to fight for justice at any cost.