The Dueling Machine by Ben Bova
(Originally published 1969. Short story originally published 1963.)
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The dueling machine, the perfect pacifier for all man’s tensions. You could enter a world of your own creation, destroy your enemy or be destroyed by him, and emerge from your mental fantasy world completely unharmed.
Dr. Leoh, creator of this ultimate placebo, could not believe that someone had now found a way to use his machine as a tool for destruction. But apparently someone had. And now this someone, Dictator Kanus of Kerak, was using the dueling machine to conquer the Acquataine Cluster.
Leoh realized it was up to him to stop Kanus before the man began a war of conquest against the Terran Commonwealth and Star Watch. But how, equipped only with an eccentric and bumbling Star Watch Lieutenant and the dueling machine itself, was he to accomplish this? And even if Leoh could discover how the dueling machine was being turned into a death machine, could he stop Kanus and his cohorts before civilization ended in an intergalactic war?