Ultra hands over the plan
Bletchley Park read the German operation order for Crete: the date, the three airfields, the strength and the order of battle. Freyberg was given it under conditions that forbade him from acting in a way that would reveal the source. He also drew the wrong conclusion from part of it, expecting a substantial seaborne landing, and disposed his reserves accordingly. Knowing everything and being unable to use it properly is one of the recurring problems of signals intelligence, and Crete is the clearest case of it in the war.