After the war, Brock worked with a Quaker relief mission to Germany and Poland, sparking his interest in Eastern Europe.
He received a second doctoral degree in history from Oxford University in 1954 with a study that resulted in the publication of The Political and Social Doctrines of the Unity of Czech Brethren in the Fifteenth and Early Sixteenth Centuries (1957), which married his interest in pacifism to his interest in east central Europe, something he would continue with studies of examples of pacifism in the region.
As a dedicated historian, Peter did not let the necessity of learning another language deter him from using primary sources on a topic that interested him.
The last book, appearing as the Vietnam War protests had revived public interest in pacifism, was "a critical and popular success".
That the scope and richness of this historical tradition can now be recognized is largely the result of Brock's sympathetic and dedicated scholarship, which was begun when pacifism was an unfashionable subject.