I did an essay about D-Day and the passage below is part of the bibliography.
The Battle of Messines, 1917
https://www.firstworldwar.com/battles/messines.htm
The Battle of Messiness helps one appreciate the anxieties aroused from memories of Gallipoli, the Somme and Passchendaele, where the British suffered catastrophic losses for gains of only yards. By some historian assessments, the most successful attack by British forces in WW I was the Battle of Messines in 1917. The battle lasted June 1-12, and involved 216,000 men of whom 24,562 became casualties. They attacked on a five-mile front and penetrated 10 miles.
Taking this battle as a starting point, the Normandy invasion from five beachheads established a lodgment about 10 miles deep on a 40 miles front. The effort required 50 days. Therefore, a WW I veteran could make an optimistic estimate of about 400,000 casualties, given he completely disregarded the fact that the D-Day landings were significantly more hazardous than any offensive attempted on the Western Front. In fact, by July 24 there were 120,000 casualties.
Thanks very much.
Churchill was responsible for the Gallipoli disaster as I recall. Thus his fears of Overlord.
Marshall stood firm on Overlord and they went ahead. Churchill later said Marshall was the greatest general of the war.