  During what Vietnamese call the American War, Alexander Grothendieck spent
three weeks teaching mathematics in and near Hanoi. Ho\`ang Xu\^an S\'inh took
notes on his lectures and later did her thesis work with him by correspondence.
In her thesis she developed the theory of "Gr-categories", which are monoidal
categories in which all objects and morphisms have inverses. Now often called
"2-groups", these structures allow the study of symmetries that themselves have
symmetries. After a brief account of how Ho\`ang Xu\^an S\'inh wrote her
thesis, we explain some of its main results, and its context in the history of
mathematics.
