What "nation" is thoreau referring to when he says, "a sixth of the population of this nation"?

Mexico

Explanation:

Henry David Thoreau was an American poet and philosopher who was notable for his book entitled, "Civil Disobedience." This book was originally published under the title of "Resistance to Civil Government." He wrote this book when he was in prison. It talks about Thoreau's disgust regarding the Mexican-American war. He argued that the people should not allow the government to do injustice. When it does, it is the role of the people to stop the government from doing it.

The nation being referred to when he stated "a sixth of the population of this nation" is "Mexico." One-sixth of the population of Mexico were enslaved. He also stated that Mexico was unjustly overrun by an invading army (the US army). So, it is just right that the Mexicans will revolt or rebel because they are being oppressed. The American people, themselves, should also stand against what their own government is doing because it is unjust.