In 2267, he was assigned a diplomatic mission to the planet Eminiar VII by Federation Central. His goal was to obtain a treaty port for the Federation. For twenty years, thousands of lives had been lost in the quadrant. Fox believed that if there was a port, those lives would not have been lost, so he was determined to get that port. He was transported to the principal planet in the quadrant, Eminiar VII, by the USS Enterprise.
After sending a message to Eminiar VII, the Enterprise received an answer from the planet. Eminiar VII issued a Code 7-10 – stay away from the planet – Fox ignored it and ordered the Enterprise to approach.
The worst happened when the Enterprise assumed orbit and became a legitimate target in the computerized war Eminiar was fighting with Vendikar. Vendikar launched a successful attack against the starship, and her crew became casualties, required to report to the disintegration stations on Eminiar so their deaths could be recorded, and Eminiar would remain in compliance with the treaty that governed the war. Kirk and Spock, on the planet below, were hostage to the crew's compliance, leaving Scotty in charge.
Scotty's caution saved the crew from a tricky attempt to get them to the surface, and his foresight prevented the ship from being destroyed by an attack from the planetary defenses. Despite these acts of overt hostility, Fox attempted negotiations, and Anan 7, High Councilor of the planet, took advantage of his credulousness. He assured Fox the attack was a misunderstanding, and invited him to the surface. Fox, as a gesture of goodwill, ordered Scotty to assume a peaceful posture – including lowering the shields. Despite Fox's threats, Scotty refused this order, insisting the shields be left up; as Anan 7 had intended to resume firing the moment the shields dropped, this act of insubordination saved the Enterprise from destruction.
Arriving at the Division of Control, Fox and his attaché were immediately rounded up and herded towards a disintegration station. Only with his death imminent did Fox begin to understand the nature of the Eminian Union. Fortunately, just as they reached the chamber, Spock (who had been able to contact the ship and discover that Fox was on the planet) intervened before the ambassador could be reduced to atoms, and took him to the war room, where Kirk was being held. After Kirk and Spock ended the war by destroying the computers that ran it, Fox offered to remain as a negotiator, to help the two rivals hammer out a peace treaty. (TOS: "A Taste of Armageddon")
Robert Fox appeared in novels including The Rift by Peter David, as the Ambassador sent to open peace talks with the reclusive Caligar race and the Rihannsu novels Swordhunt and Honor Blade at the failed peace talks with the Romulans.
In the DC Comics "Who Is Enigma?" issue he encountered his daughter as he tried to save peace negotiations between the Klingon Empire and the Federation, and he assisted in the evacuation of reluctant inhabitants of the doomed planet Aleriad in the FASA RPG scenario Denial of Destiny.