Was D-Day history's greatest gamble?

Was D-Day the greatest gamble in history?

No: it was extremely dangerous for participants, but Allied preparation made it a calculated operation rather than the largest strategic gamble.

The Allies assembled dominant air and sea power, large ground reserves, specialised landing craft, and purpose-built engineering vehicles to reduce uncertainty. That preparation does not diminish the danger faced by troops crossing exposed beaches, but it makes the operation less reckless at the strategic level. Overlord differs from campaigns that crossed into the unknown with little logistics or support, including Wolfe at Quebec, Nelson at Copenhagen, and several ancient examples. Individual danger at the beaches did not make the high command equally careless with the strategic variables.