Germany's Offensive against Poland started in the early Hours of September 1st, officially at 5:45 as stated in Hitlers famous Speech at the Reichstag later that Day. So normally, the Moment Brian and Stewie arrive at Warsaw, Germany would be already at War on Poland.
While escaping the German roadblock dressed in clerics, Mort Goldman's collar - which has no attached rabat - is removed by the German officer then is seen again back on his neck while in the motorcycle sidecar.
When Mort goes into Stewie's room and finds the time machine, there is a large sign on the bedroom door saying: 'Stewie's room, keep out Brian.' This sign is not present when other characters enter or exit the room.
When Stewie asks for Mort Goldman in the Warsaw Temple, a Jew with a beard and hat is sitting at the front on the first seat the right hand side. The shot where it first showing Mort at the Warsaw Temple, he is at the front on the first seat of the right hand side.
Brian, Stewie, and Mort time travel to a wedding in Warsaw Poland on September 1, 1939, whereupon they encounter invading German troops. Although the Germans crossed the Polish border on September 1, they didn't reach Warsaw until September 29.
Mort, Brian and Stewie arrive in London in early September 1939 and the city is filled with bombed and smoking buildings. England declared war on Germany on September 3, 1939 and Germany did not bomb England until July 1940, the Battle of Britain.