Final answer:
Stephen Ambrose describes D-Day as the greatest achievement of the American Republic in the first half of the 20th Century due to the operation's scale, the success of the amphibious assault, and the significant impact it had on the outcome of World War II.
Step-by-step explanation:
When Stephen Ambrose refers to D-Day as "the greatest achievement of the American Republic in the first half of the 20th Century," he is highlighting the monumental scale and success of Operation Overlord on June 6, 1944. This operation was an amphibious assault involving over 160,000 troops crossing the English Channel to invade Nazi-occupied France. The complexity, coordination, and sacrifice involved in carrying out the largest amphibious assault in history earned it a place of high regard in American history, symbolizing the power and effectiveness of the American military and marking a turning point in World War II.
By securing the Normandy beaches against heavy German defenses, the Allies established a crucial beachhead. This allowed for the landing of additional troops and heavy equipment, ultimately leading to the liberation of Paris and easing the pressure on Soviet forces in the east. The planning and execution of D-Day required an unprecedented level of international cooperation and demonstrated the technical and logistical capabilities of the U.S. military. It exemplified the American commitment to the fight against tyranny alongside their British and Canadian counterparts.