His first campaign was against slugs who had crawled into an old woman's house. She asked the Sontarans to dispose of the Slugs using salt guns. The Twenty-First Sontaran Legion was called in to destroy the slugs, but the entire battlegroup was destroyed by the Slugs in a short conflict, the occurances of which go unspoken as all the combatants died. Flats was found dead by a radiator, it seems the heat melted his plastic suit and his skin peeled off, revealing the lack of a brain inside.
And thus ends the tragic parable of Field Marshal Flat. The morale of the story is: Don't be a Sontaran.