Complex sentence: This enabled the British at Bletchley Park, who had been unable to break German Enigma ciphers, to develop their "Ultra" operation that General Dwight Eisenhower characterized at war's end as having been "decisive" to Allied victory.
Split into simpler sentences: This enabled the British who had been so far unable to break German Enigma ciphers at Bletchley Park, to develop their "Ultra" operation. At the end of the war, General Dwight Eisenhower characterized this operation as having been "decisive" to the Allied victory.