In 1632 he joined the armed forces under Wallenstein, who, in view of the Swedish progress, had been again appointed Imperial commander-in-chief by Emperor Ferdinand II. Butler quickly gained the trust of the "Generalissimo" and was appointed Colonel of a dragoon regiment. While the Imperial army fought against the Swedish at Zirndorf and Lützen, Wallenstein's envoys were in secret discussions with the opponents, which led to accusations of making peace with the King of Sweden without the consent of the Emperor. When Wallenstein's treachery was discovered in 1632, it was Butler who saved the Empire and arranged his assassination in 1634. He together with the officers John Gordon and Walter Leslie had the plot executed on 25 February, after the murder of Wallenstein's confidants Christian von Ilow, Adam Erdmann Trčka von Lípa and Vilém Kinský.