The books that fell off my bookshelf into my hands most recently were coincidentally of a type. One is a tattered copy of Lincoln’s Generals, by T. Harry Williams (1953), which I picked up free at the end of a garage sale, and the other is Sir Basil Liddel Hart’s History of the Second Word War (1970), which I've had around the house for at least 20 years.
The Allies themselves made mistake after costly mistake. After the Belgians failed to blow up a few key bridges, the numerically superior French armies panicked at the Nazi tank columns invading from the north (instead of dashing themselves to pieces on the Maginot line). The Brits moved a weak force into Greece that only served to attract the Nazis' wrath. In Crete, the New Zealanders ceded the airport to a smaller attacking force and failed to counterattack while they had the manpower advantage – dooming them to defeat and capture.
Several times the British failed to chase Rommel out of Africa when they had the chance. And by building up their defences in Egypt, the Brits doomed Singapore, a more strategic asset.