Notes by RWF:

Adolf Hitler's birthday was celebrated throughout the Third Reich and occupied territories, often with speeches and parades in the major cities. I do not know the date of this triumphal parade through Warsaw.

The Warsaw street pictured is one which escaped the Nazi air blitz and ground assault. The pavement seems to have been scrubbed further by a Nazi-era photographic shop. The lead automobile's headlights appear blacked out in the photograph, perhaps so they wouldn't visually distract from the rest of the scene.

At this point at least, the bystanders consist of more German troops than Polish civilians.

Karl Hennig was a major stamp dealer in Hamburg with Nazi Party connections. This is a souvenir card of a type sent in quantity to his dealership; or possibly a postwar forgery of the cancellation by Hennig using original materials.

Philatelists will notice that the prewar postage stamp of Poland is overprinted with the Nazi eagle, a 2-zloty denomination, and the authorizing agency, the General-Gouvernement. This agency, giving its earlier name in full, is the Generalgouvernement für die besetzten polnischen Gebiete, or the General Government for the Occupied Polish Territories, based in Cracow. This controlled, under Adolf Hitler, that region of conquered Poland which had not been directly annexed by Germany, nor occupied by the Soviet Union.

General Government of Poland at Wikipedia
  

  
On my rediscovery of this postcard:
Oops — Hitler!
A Surprise in Context
by R. W. Franson

Germany at Troynovant
Imperial Germany, Third Reich
Prussia, Bavaria, Austria
history, geography, literature
  

  
Postcards by Title
flat-carded Scenes of Passage:
in the older manner sent & reviewed in Private
though carried by the Public post

Warfare at Troynovant
war, general weaponry,
& philosophy of war