Final answer:
The poem "September 1, 1939" explicitly criticizes Nazi Germany and the actions of the German Reich.
Step-by-step explanation:
The institution that "September 1, 1939" explicitly criticizes is the German Reich or Nazi Germany, which is mentioned in the provided information. The excerpt mentions President Roosevelt giving speeches inciting the American public against Germany, and it also mentions the rise of the Nazi regime and the Frankfurt School's relocation due to it. These references indicate criticism towards Nazi Germany in the context of the provided information.