In Europe: Omaha Beach was an avoidable disaster, the Normandy hedgerows were a suprise, B-17s killed hundreds of GIs in Operation Cobra, the Falaise Gap allowed the German 7th Army to escape, the Sherman tank was inadequate, etc., etc.
Wars are difficult, criticism is easy.
As General Tommy Franks said in his book, while planning the liberation of Afghanistan and Iraq:
"The enemy gets a vote"