In 2017, Colonel Brabbit led the US forces stationed in Turmezistan. A day after a pyramid appeared overnight in the middle of a war zone, he spoke with Secretary General of the UN and Bill Potts over the phone. Don Brabbit clarified that the pyramid is 5,000 years old, despite it recent appearing.
As Secretary General, the Twelfth Doctor, Bill and Nardole arrived at Turmezistan, Brabbit showed them the pyramid. He represented the US Army during the meeting with Ilya Svyatoslavovich and Xiaolian. During the meeting, according to the sonic sunglasses, he was 177.6 cm tall, weighed 141.0 pounds, had a resting heart rate of 69 BPM and a body temperature of 38 degrees Celsius. Following the Doctor's advice to co-ordinate attacks, Don Brabbit orders an aircraft to bomb the pyramid. However, the attack was unsuccessful. Brabbit was among the people who entered the pyramid for negotiation. After the Monks killed Secretary General, Xiaolian proposed other commanders to promise that they will not fight against each other. Brabbit was the first one to agree and shake hands with her. The Doctor understood that the possible disaster that could force the humanity to surrender to the Monks is not the World War III, so he made all the top-secret intelligence documents in the world available for online search engines, to the displeasure of the colonel, and asked everyone to "sit down and google".
After searching for possible disasters, Don Brabbit proposed to surrender to the Monks. His plan was to live "to fight another day". The Doctor didn't like the plan, but Xiaolian and Ilya Svyatoslavovich agreed. Together they entered the pyramid. The Monks killed all three of them by disintegration, because they acted out of strategy, not consent. (TV: The Pyramid at the End of the World)
Behind the scenes
In the politically absurdist original draft of the story, when it was called We Dare Not Go a-Hunting, Donald Brabbit was a Trump-like figure who faced off against analogues of Jeremy Corbyn, Vladimir Putin, and Kim Jong Un for the favour of a sole Monk. In the second part, For Fear of Little Men, Brabbit died when he was killed by his own toupee.
Though Brabbit is called colonel (grade O-5), the actor playing him wears the rank of a 4 star general, (grade O-9) which may be more appropriate to the high level role he plays in these scenarios. To illustrate the absurdity of this dialog error, the highest official rank (and grade) is 5 star general, aka General of the Army (O-10), but there has not been one of those in generations.