Matthiessen is best known for his fiction work, his writings on the natural world and his writing and activism on environmental causes.
In the Spirit of Crazy Horse is one of the finest examples of a book that exposed historical and political injustice and gave force to a political campaign that continues to this day.
In In the Spirit of Crazy Horse, Matthiessen documents the massive number of crimes committed against Native American people and the genocidal nature of the US authorities in their destruction of Native cultures and people. He documents a long string of historical injustices, ranging from mass slaughter to the takeover and reapportionment of sacred or Native lands, including the forced appropriation of Indian lands for mining use.
Peltier, an Ojibwa-Lakota from Turtle Mountain, North Dakota, was one of the three young Indians involved in a shoot-out with the FBI at Oglala on South Dakota’s Pine Ridge Reservation in June 1975, which resulted in the death of 3 men, including 2 FBI agents and an Indian. Pelteir and 3 others were later charged with the murder of the two FBI agents.
Matthiessen's book was so controversial its publication was delayed for decades due to legal challenges by the US authorities and the FBI, which cost Matthiessen and his publisher $2 million in legal fees.
In 1969 Matthiessen also wrote a book length profile of Ceser Chavez the legendary union organizer for farm workers in the US. The book was titled Escape if You Can: Cesar Chavez and the New American Revolution.
Matthiesson continued to write about Chavez, quoting him in this profile article:
When we are really honest with ourselves, we must admit that our lives are all that really belongs to us. So it is how we use our lives that determines what kind of men we are. It is my deepest belief that only by giving our lives do we find life. I am convinced that the truest act of courage, the strongest act of manliness, is to sacrifice ourselves for others in a totally non-violent struggle for justice. To be a man is to suffer for others. God help us be men.”