On June 6, 1944 – exactly 75 years ago – 156,000 men took part in the Allied invasion of Normandy in France. They suffered more than 9,000 casualties on that bitterly fought day, but their success sounded the crack of doom of the Axis powers in Europe – and guaranteed that the destruction of imperial Japan would be next, ending World War II.

A question I’m often asked as a historian and defense policy analyst is: Could we do D-Day again? Would the sacrifice, resources and effort required on D-Day still be there if we found ourselves in a similar crisis with everything on the line?

The reality is, we needed four things to bring off the successful invasion of Europe that June 6.

WORLD LEADERS INCLUDING PRESIDENT TRUMP GATHER TO COMMEMORATE 75th ANNIVERSARY OF D-DAY LANDINGS

The first was the intrepid men and women of four nations – Britain, Canada, France and above all the United States – who were willing to risk everything to carry out the mission.

The second was what the military calls "domain dominance" over sea and air in the English Channel and Atlantic, which took three long years after the Battle of Britain to acquire by crushing Luftwaffe airpower over Europe and the German U-boat menace.

Third, we had a defense industrial base unlike any in history. America created the tanks, ships, planes, and war material enough to arm the U.S., Britain and the rest of the free world – and give the Soviet Union the weapons and resources it needed to keep the bulk of the Nazi war machine tied down in the East.

As I recount in my book “Freedom’s Forge,” by 1944 – when American soldiers were landing on Omaha and Utah beaches in Normandy – American factories were producing a warplane every five minutes, eight aircraft carriers a month, and 50 merchant ships a day.

Fourth and most importantly, Americans had a sense of unified spirit that refused to accept anything less than victory. Everyone from the soldiers and sailors on Omaha and Utah Beaches; to workers in the aircraft factories – where 42 percent of the workforce was made up of women; to kids collecting pots and pans for aluminum drives, knew that sacrifice and mutual support were the key to eventual success.

So do we have what it takes to do it again?

With the men and women of our armed forces and those of our closest allies, the answer I believe is an unqualified “yes.”

The reality is that if we ever found ourselves in a protracted conflict with a China or Russia – not a half-decade conflict like World War II, but one lasting just a few months or even weeks – our men and women in uniform would be in serious jeopardy.

As for domain dominance, the “yes” must be more qualified. Over the last decade and a half, potential foes like Russia and China have built their entire military strategy and arsenals around denying us that dominance. This is true in Europe and the Middle East with Russia’s advanced S-400 anti-aircraft system, and in Asia with China’s supersonic anti-ship missiles and growing fleet of attack submarines.

But we could still prevail if that third requirement is in place, namely a robust and agile defense industrial base.

A report from the White House released last September says we are in serious trouble in that regard.

Since 2000, the report said, the entire defense industrial base has shed more than 20,500 U.S.-based manufacturing firms. Much of the work they used to do has been sent overseas, including to China – as if our fighting men on D-Day had had their helmets and knapsacks manufactured in Nazi Germany.

Today the Navy currently has only one firm manufacturing and refurbishing shafts used by both surface ships and submarines.

Only one production line produces all the large-caliber gun barrels, howitzer barrels and mortar tubes used by our armed forces.

Of course, this de-industrialization of our armed forces is nothing new.

The U.S. machine tools sector – essential for making anything that requires manufacturing – has been shrinking since at least the 1980s, while China has been surging ahead from 15 percent of global machine tool consumption in the mid-2000s to 40 percent by 2011 – and still growing.

In 1961, the same year President Dwight Eisenhower was warning us about a “military-industrial complex,” 15 defense companies were in the top 100 of the Fortune 500.

In 2015 the top 100 listed only four aerospace and defense companies, with much of their revenue coming from non-military commercial activities. General Dynamics – No. 15 in 1961 – barely made the bottom of the list, at No. 100.

The reality is that if we ever found ourselves in a protracted conflict with a China or Russia – not a half-decade conflict like World War II, but one lasting just a few months or even weeks – our men and women in uniform would be in serious jeopardy.

Don’t believe those who say we won’t ever have to fight a long war again, or that robots will win it. Iraq and Afghanistan give the lie to the first assumption, and robots require machine tools and a manufacturing base that is strong and healthy.

The White House has taken serious steps to correct the problem. These include announcing a national strategy for restoring American leadership in advanced manufacturing. But we need more radical solutions if we are going to build the Freedom's Forge and Arsenal of Democracy for the 21st century.

That’s where the fourth factor, a unified spirit, comes in. Some say we’ll never have that sense of unity that animated the Greatest Generation. They say our polarized politics and the advent of our “snowflake generation” make that kind of sacrifice unlikely.

Others, like me, aren’t so sure. America in 1940 had been pretty divided, too, until the Japanese attack on Pearl Harbor finally pulled us all together.

If the initial response to the Sept. 11, 2001 terrorist attacks is any indication, anyone betting against that unified American spirit had better hedge those bets.

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In any case, on this June 6 it’s important to remember that D-Day didn't just happen as a one-off. It took years of cumulative effort and coordination, with government, business, technology and the American people all working together.

We'll need that coordination if we ever put our men and women in harm's way again, as we did that day in June 75 years ago on the beaches of Normandy.

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