You can read about it here
It sounds cynical, cold-blooded to say it, but the real lesson of D-Day..the reason the invasion succeeded, was because commanders were willing to sacrifice troops to complete the mission. Utah Beach was a near disaster, they were almost ready to pull the troops off..instead..they were sent forward...as we read about Bergdahl..and the supposed doctrine of "leave no man behind"..it's interesting to look at how that would have played out in WW II.. The invasion troops were taught one thing...it was drummed into them...do NOT stop to help your buddy if he's shot...keep going forward..get off the beach..." That's why we won..
“There are two kinds of people who are staying on this beach: those who are dead and those who are going to die. Now let’s get the hell out of here.” - Brigadier General George A. Taylor.
Note in “The Longest Day” that quote was attributed to Norman A. Cota, which is incorrect.