Why was thoreau put in jail? what were his feelings about the government while he was in jail?

The first man to practice passive resistence, or "civil disobedience," as he coined the phrase, Henry David Thoreau chose to spend a night in jail rather than pay what he considered an unfair tax. Thoreau acted this way in order to protest the United States's war with Mexico and slavery for which the monies from the tax would proceed. 

After his incarceration, Thoreau wrote his seminal essay which examines the relationship between government and the individual. Thoreau's thinking was much more in line with that of the Founding Fathers than it is today in this era of big government. For, he felt that the best government is the one that governs least. That is, it allows men to exercise their individual rights to exercise their own wills and do what is best. After all, Thoreau argues, government is not supposed to dictate to people what they must do; instead, the government should do the bidding of the people. Thoreau believed that the people would not have chosen to go to war with Mexico:

fearlessness is your answer