In a World where Barry Allen never had to experience a Second World war he had a clear future until Reverse Flash discovers a ripple in the time paradox and makes his way too the start of a new timeline with a different future. He ends up in Czechoslovakia.
S1.E4 The Blitz
He discovers he is in Czechoslovakia 1938 due to a newspaper. Barry still hasn't found the reverse flash yet. He doesn't want the war to change because everything could go wrong and his future life may end up different. Barry and the reverse flash runs to get away. Barry ends up having to find him again in Britain.
The date is July 7th, barry had just found the reverse flash. The Battle of Britain had been 3 days out and barry knew he would try to mess it up. When nazi germany sends the bomb the reverse flash has planned not to drop the bombs.
S1.E6. Pearl Harbor
Barry had found out that the reverse flash was going to stop the bombs from hitting the Raf. This would mess up the war so he had to make sure that the bombs did launch and did hit where they had planned to. If the reverse flash had succeeded in any way possible this would mess up the timeline.
Barry tracked Reverse flash to the shore but then lost him because he took of for Japan. Barry eventually made his way to Hawaii. Reverse Flash shows up and tries to confront Barry. The reverse flash had told Barry of a man named Hitler and the power he has. They hear planes. This was alerting because this was outside of protocall. They get closer and are recognized as The Kamikaze.
This is a surprise attack due to the men being caught off guard. The torpedoes hit the ships and caused them to sink and many men inside are drowning. Barry tries to save some men but the ships. The Flash travels around the island helps the survivors as the news reaches the White House. The next day FDR addresses the US that we are now in War.