Tierney enlisted in the Royal Air Force in July 1941 and served in Bomber Command until 1946, attaining the rank of Flight Lieutenant.
He next served for a year as a navigation instructor and then returned to operational flying and completed a second tour of sixty missions on Mosquitoes with 105 Squadron of the elite Pathfinder Force.
[citation needed] He then began graduate work under the guidance of Walter Ullmann a learned Austrian scholar who was primarily responsible for introducing the study of medieval canon law to English historians.
Tierney argued that this teaching was not just an unfortunate aberration, as modern Catholic theologians came to suppose, but was grounded on an established body of constitutional law that had been formulated in earlier canonistic writings.
[7] Although the book was primarily of interest to medievalists it also attracted the attention of some of the expert participants (periti) at Vatican Council II (1962–65) who found in the early sources support for their own vision of the church.
[9] Since the book called into question the validity of the decree of 1870 it attracted a substantial body of commentary, some favorable, some highly critical.
In 1974 Tierney engaged in a published debate with Alfons Stickler, the Prefect of the Vatican Library (and a future cardinal).
[10] Although the subject was very sensitive and the two parties held diametrically opposed views, it was noted that the debate was conducted without rancor and with courtesy on both sides.
[12] Some thirty years after Tierney's book was published a summing up in the New Catholic Encyclopedia mentioned various criticisms of the work but added that "Most scholars recognize that Tierney correctly located in the late 13th and early 14th centuries the first discussions of papal infallibility" and that, as regards other disputed points in his work, "the discussion continues."
[19] He received research grants from the Guggenheim Foundation (1955, 1956), The Institute for Advanced Study, Princeton (1961) the American Council of Learned Societies (1961, 1966), and the National Endowment for the Humanities (1977, 1985).