that Germany could be stopped
If you mean, "What crucial lesson was learned in the battle of Britain?" the best lesson learned is that the outnumbered Royal Air Force (RAF) ("Never before have so many owed so much to so few!") was able to successfully defend England primarily due to the Brits having the German code machine "Enigma."
With it the British radio intercept operators were able to know in advance the general location and times of the Luftwaffe attacks. The new invention, RADAR, then provided precise information about the time and location of the attacks. This allowed the RAF pilots to save time and "petrol" so they had only to go into the air immediately prior to an air attack. Thus they were able to rest and use their meager resources prudently and effectively. And the Battle of Britain was won, certainly by the Air Force but also by the radio intercept operators. The failure of the Luftwaffe meant Hitler could not invade England. This was the first of his major blunders. (The next was his invasion of Russia.)
Incidentally, the "Enigma" was also instrumental in solving the German Undersea Boat (U-boat) problem in the North Atlantic. English intercept operators were able to listen to German Headquarters assigning positions to the submarines throughout the Atlantic. US and English Anti-Submarine Warfare (ASW) forces were able to seek and destroy the submarines based on that information.
that Germany could be stopped
If you mean, "What crucial lesson was learned in the battle of Britain?" the best lesson learned is that the outnumbered Royal Air Force (RAF) ("Never before have so many owed so much to so few!") was able to successfully defend England primarily due to the Brits having the German code machine "Enigma."
With it the British radio intercept operators were able to know in advance the general location and times of the Luftwaffe attacks. The new invention, RADAR, then provided precise information about the time and location of the attacks. This allowed the RAF pilots to save time and "petrol" so they had only to go into the air immediately prior to an air attack. Thus they were able to rest and use their meager resources prudently and effectively. And the Battle of Britain was won, certainly by the Air Force but also by the radio intercept operators. The failure of the Luftwaffe meant Hitler could not invade England. This was the first of his major blunders. (The next was his invasion of Russia.)
Incidentally, the "Enigma" was also instrumental in solving the German Undersea Boat (U-boat) problem in the North Atlantic. English intercept operators were able to listen to German Headquarters assigning positions to the submarines throughout the Atlantic. US and English Anti-Submarine Warfare (ASW) forces were able to seek and destroy the submarines based on that information.
Lesson learned: Britain rules the seas.
That air superiority must be achieved before a land invasion can be successful.
That this was not going to be a short and glorious war.
The necessity for air superiority.
Never attack a well defended position on the high ground with numerically inferior forces.
That Germany in fact, COULD be stopped!
Lesson learned: Britain rules the seas.
that hilter's advances could be blocked
That air superiority must be achieved before a land invasion can be successful.
History's first clash of carriers. Lesson learned: How to fight carrier battles
That this was not going to be a short and glorious war.
A Lesson to Be Learned was created on 1992-09-16.
The necessity for air superiority.
Never attack a well defended position on the high ground with numerically inferior forces.
The homophones for something learned are "taught" and "taut." The homophones for to reduce are "adduce" and "deduce."
"Anthrax" performs the song "A Lesson Never Learned."
Another term for lesson learned is "takeaway" or "key insight."