It's still the largest amphibious invasion ever. The outcome's implications were also huge.

As Capt. Henry J. Hendrix, chief historian of the U.S. Navy, puts it in a new PBS "Nova" on "D-Day's Sunken Secrets":

"What is at stake was nothing less than the history of Western civilization. It was an all-out gamble. It was pushing all your poker chips onto the center of the table."

That inherent D-Day uncertainty is too often overlooked.

So as you pause to rightly remember the awesome bravery of the Allied forces who stormed the Normandy beaches 70 years ago today, also remember this:

On June 6, 2014, we know that the Allies emerged victorious on D-Day. But at the dawn of June 6, 1944, the Allied warriors didn't know what would happen on - or after - D-Day.

They knew, though, that they likely would suffer massive casualties while wading into the teeth of the Nazi war machine - as they did. The Allies lost at least 9,000 killed or wounded in that first day (some casualty estimates are significantly higher).

And even those who survived "the longest day" knew there would be much more fierce fighting to follow - again, with no guarantee of final victory.

That enlightening "D-Day: Sunken Treasures" edition of "Nova," which first aired last week and again on Wednesday night, will air one more time at 1 a.m. Sunday on WITV.

It shows high-tech tracing of a vast volume of sea-bottom D-Day residue - ships, landing craft, planes, guns, even tanks.

It features first-hand accounts from Allied veterans who lived not just beyond that D-Day hell on earth but into old age. Their recollections are chilling, touching and inspiring.

That "Nova" also includes military experts' analysis of the D-Day fighting, which ebbed and flowed like the tide - a harrowing reminder of not just the terrible price paid in blood, but the uncertainty of the result.

So no, our D-Day heroes couldn't be assured of winning - or surviving - the epic battle that awaited them.

They were, however, vividly informed of the immense stakes in this stirring send-off from Allied Supreme Commander Gen. Dwight D. Eisenhower:

"Soldiers, Sailors and Airmen of the Allied Expeditionary Force! You are about to embark upon a great crusade, toward which we have striven these many months. The eyes of the world are upon you. The hopes and prayers of liberty loving people everywhere march with you. In company with our brave Allies and brothers in arms on other fronts, you will bring about the destruction of the German war machine, the elimination of Nazi tyranny over the oppressed peoples of Europe, and security for ourselves in a free world.

"Your task will not be an easy one. Your enemy is well trained, well equipped and battle hardened, he will fight savagely.

"But this is the year 1944! Much has happened since the Nazi triumphs of 1940-41. The United Nations have inflicted upon the Germans great defeats, in open battle, man to man. Our air offensive has seriously reduced their strength in the air and their capacity to wage war on the ground. Our home fronts have given us an overwhelming superiority in weapons and munitions of war, and placed at our disposal great reserves of trained fighting men. The tide has turned! The free men of the world are marching together to victory!

"I have full confidence in your courage, devotion to duty and skill in battle. We will accept nothing less than full victory!

"Good Luck! And let us all beseech the blessings of Almighty God upon this great and noble undertaking."

And on this 70th anniversary, let us honor those whose indispensable sacrifices won that D-Day victory.