The actor took aim at the Army in a bid to stop officials using live monkeys in chemical attack training exercises at the Aberdeen Proving Ground.
Harrelson, who landed an Oscar nomination for his portrayal of an Army captain in The Messenger, wrote a letter to Odierno on behalf of People for the Ethical Treatment of Animals (PETA) a month ago (Sep11).
In the correspondence, he urged the general to "stop this crude exercise," adding, "I'm sure you agree that our military personnel deserve state-of-the-art training and that our country deserves to be respected for its civilised treatment of animals."
His letter has had the desired effect - the U.S. Army has confirmed it will discontinue cruel monkey laboratories that were part of Aberdeen Proving Ground's chemical casualty training courses.
PETA Vice President of Laboratory Investigations Kathy Guillermo says, "We are delighted and relieved that the U.S. Army has done an about-face and decided to use only modern, human-like simulators at all of its bases.
"This is an enormous victory for the animals who will no longer spend their lives in cages and be intentionally poisoned."