All the broken trees represented all the countries the Nazis had destroyed during World War II. The bird that was destroying all the trees had a Nazi sign on it, so the bird represents the Nazis. Finally, the last tree that was standing was America because the bird that was on top of the tree wore an Uncle Sam hat. The quote said “Hohum! When he’s finished pecking down that last tree he’ll be quite likely tired” which also means that America doesn’t think that Germany will attack because they have already attacked so many countries. So in real life what happened was that Germany never actually attacked America, but other countries in the axis (Japan, Germany, and Italy) attacked America, like Japan in Pearl Harbor. America assumed Germany would never attack due to them being tired after attacking so many countries, but never imagined that Pearl Harbor would be attacked by Japan.
Hope this helps. - M
