Paul Brenner is an agent with the State Department's Diplomatic Security Service (DSS).
As an agent, it is his job to go the trouble spots of the world where American diplomats must also travel to help keep the danger from destroying vital diplomatic missions. It is one of those thankless jobs that if an agent does correctly time after time, no one thinks about it. If he messes up or if the enemy is just a little better or luckier, his career is likely over, assuming he is alive to care.
Brenner had not always been an agent with the DSS. He had planned on a long and interesting career in the US Army's Criminal Investigation Command and it is in that role that we first see him. A veteran of far too much action in Vietnam, Brenner has advanced through the enlisted ranks before becoming a Warrant Officer (rank of CW3). Far more important in his role with the CIC is power that is given him in order for him to do his job - the power to arrest any one, regardless of rank or position. What makes that a tad touchy, however, is the fact that prior to the actual arrest, Brenner must follow the discipline of any member of the military. This means a fine line must be walked.
This is nothing new to Brenner. He has had people, both the enemy and 'friendlies', try to kill him in war and in peace and his brassy nature and somewhat cynical tone conveys this. He knows that every service-person, regardless of sex or age or rank or position, puts their pants on one leg at a time and he knows that any one of them can and might step over the line. It is his job to catch them when they do.
At least it is in the first recorded case. In the second book detailing his activities, Brenner is only just recovering from the political and personal fallout for not having played ball like it was suggested he should. He demanded to know the truth and he got it. He got 'allowed' to retire early as a result. So he is available when a trusted former commander asks for help and that request takes him back into one country he had never expected to see again, a country where he had nearly died several times and one that had many bad, but also many good, memories; Vietnam.
The activities of this mission threw Brenner into a world of shadows where nothing he saw or heard could be trusted and no one had his back. This sort of predicament was not novel to the highly trained detective as he had been undercover many times before but it was always in amongst his own people, speaking his native language, and with the ability to step out of character if truly forced and call upon the might of the US Army for help. In this Vietnam mission, there was no stepping out and definitely no one to ask for help.
It was after this second adventure that Brenner realized he still had a life to lead and that life had to be one where the stakes were a lot higher than just another paycheck. He knew he could get a security job anywhere for virtually anyone. He chose the DSS and he chose to make every day he went to work a day he might not survive.