The stainless steel rat gets drafted

Of all of Harry Harrison's many great books and series, the Stainless Steel Rat series with Slippery Jim DiGriz stands apart. In this book, Jim is out for revenge against Captain Garth - neither the rank nor the name are real - who is responsible for the death of Jim's mentor, The Bishop. Nothing is ever simple, a pretty straightforward desire to throttle Garth gets morphed into Jim joining - albeit through a back door - the army of Garth's home world, a rapid rise and fall of rank of the self promoting type, and ends with Jim fighting against the army to save a pacifist planet and people whom Garth would enslave and kill without mercy. Plenty of action and the reader isn't ever bored.