(The first book in the Interstellar Patrol series)
A novel by Christopher Anvil
The starship crew was stuck on a planet where the well-meaning schemes of social engineers had created a nightmare of battling gangs. So they pretended to be the Royal Legions from a distant star kingdom pursuing an all-powerful villain hiding on the planet. Then the real Royal flagship showed up.
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Used availability for Christopher Anvil's Interstellar Patrol
September 2004 : USA Mass Market Paperback
March 2003 : USA Paperback
December 2013 : USA, Canada, UK Kindle edition