Under the cover of naval shell fire, American infantrymen wade...

Under the cover of naval shell fire, American infantrymen wade ashore from their landing craft during the initial Normandy landing operations in France, June 6, 1944.  Credit: AP/PETER J. CARROLL

I recently visited the beaches of Normandy and was awe-struck at the scale of what took place there 78 years ago. The sheer horror it must have been is hard to capture in words, but the scars are still visible.

Dozens of massive craters dot the fields. The landscape is interspersed with German fortifications reinforced with concrete two meters thick. Huge tangles of metal debris, part of the harbor constructions used to ferry in half a million troops and cargo, are still casually strewn across the beaches and protruding from the water, rusting just off quaint French coastal towns.

It was a massive operation. More than 1,000 aircraft dropped 23,000 paratroops behind German lines in the dark amid incoming fire just after midnight. At dawn, 73,000 Americans joined about 80,000 other allied troops to storm the beaches along 50 miles of coastline.

The morning was cool and the water colder. Soldiers lugging nearly 75 pounds of gear each forced their way through rough seas only to reach a treacherous shore where they would then navigate stakes, metal girders and land mines, all while Nazis rained fire down upon them. Many would die there or farther inland as the battle continued.

How do you motivate anyone to do that? And what would compel anyone to ask so many people to?

These questions were on my mind when I heard of the death of Bradford Freeman, the last living member of the Band of Brothers company thatparachuted into Normandy on D-Day. We’ve recently lost Hershel “Woody” Williams as well, the last surviving World War II Medal of Honor recipient.

More than 16 million Americans served in World War II. But we’re starting to lose the very last veterans of that war, one of the few we’ve fought in our history that we can confidently call just.

What lessons should we be learning from the Greatest Generation before they’re gone?

I often wonder what my grandfather, a U.S. Marine who served in the Pacific, would say. His faded photos in jungle scenes were intriguing but revealed little, and he said even less about it. He died when I was young, long haunted by what they called “shell shock” back then.

I imagine that he, and many others of his generation, would tell us that war is so horrific that you should avoid it at almost any cost. But if it’s something so vital that you simply must fight for it, you better be ready to give it your all.

In many ways, that is the story of America’s engagement in World War II. We entered reluctantly — forced by the circumstances of a direct attack on our homeland. We responded with everything we had, and it transformed a culture and economy for generations.

The Normandy invasion was the ugly, painful crowning moment of that response. The logistics were mind-boggling and the plan audacious. But nothing less could turn the war around and save Europe, and the world, from the march of totalitarianism.

In many of the wars we have fought since, we have strayed far from these parameters. We have fought wars that were not essential and even damaging to American security and prosperity. It shouldn’t be surprising that we tended to lose these wars of choice. It’s harder to fight effectively if it isn’t clear what you’re fighting for.

So what lessons might that offer us today? The war in Ukraine is a good test case. The fight came to the Ukrainian people — they had little choice. We do not face a similar existential threat now, and thus have wisely avoided expanding this awful war beyond Ukraine by getting directly involved in the fight.

But sometimes simply not entering a war isn’t enough. The United States and our allies must at times take the necessary steps to deter such aggression. That is the case with Ukraine, and that means using all the tools available to us short of joining the war to help Ukraine succeed.

If Putin’s violent land grab in Europe succeeds instead, the international system we helped build and benefit from will be deeply undermined. That system checks aggression and helps keep us safe, prosperous and free. The threat to us directly might not be grave enough yet to risk American lives, but we’d be wise to prevent it from reaching that point while we’re still able.

Ukraine’s Greatest Generation is being forged today as they fight and die for their country’s freedom. Their fight against aggression and authoritarianism echoes the Allies’ fight in World War II. By supporting their effort, we can hopefully avoid having to ask another generation of Americans in the future to suffer what Bradford Freeman, Woody Williams, my grandfather and millions of others did.

This is important for Americans to remember today as we contend with economic struggles and domestic strife at home. If we continue to rigorously invest in helping Ukraine with diplomatic, economic and military assistance efforts now, we might not have to fight aggression and authoritarianism ourselves in the future.

Elizabeth Shackelford is a senior fellow on U.S. foreign policy with the Chicago Council on Global Affairs. She was previously a U.S. diplomat and is author of “The Dissent Channel: American Diplomacy in a Dishonest Age.”