Second Night of Summer

Published in Galaxy Science Fiction, December 1950. Collected in Agent of Vega
Vegan agent 'Grandma' Wannatell is sent to the planet of Noorhut in preparation for an invasion by a very nasty alien race known as the Halpa. Her nearly impossible task is to not only stop the invaders from succeeding, she must do so without letting the populace know the danger they are in. If she fails, there are several Vegan Confederacy battleships nearby ready to 'sterilize' the planet of all living things.