Welcome, Nicola! It always picks up a forum to have you visit! Please post some images from your library to show why it is called the 'Truman.'
The Conference of Potsdam took place in July-August 1945 between Truman, Staline and Churchill (then Attlee). At this conference, the political and economic principles governing the control of Germany alter its capitulation were determined. http://catalog.antiquorum.com/catalog.html?action=load&lotid=64&auctionid=122
Hint hint, see 40 Down at: https://omegaforums.net/threads/just-for-fun-a-crossword-puzzle-for-you-Ωf-–-ug-style.34196/ (thanks for indulging the shameless own-thread-plug).