YPRES, Belgium - Children now scamper through the remaining trenches on Hill 62 in Flanders fields, where Brits endured a World War I hell of mud and shelling.

Lowland water still oozes through the thick clay of the soil within and beyond the trenches, and conjures scenes of men charging over the top and braving German fire, only to sink to their death in the ooze.

Today's politics in America and Europe require us to think again about those battles.

April 6 marks the 100th anniversary of the entry of U.S. troops into the Great War. (They fought mostly in northeast France, but some arrived in Flanders near the end of the conflict.) That war's uneasy end led two decades later to World War II, with combined casualties that dwarfed anything the world ever imagined. But the last 70 years has produced an unprecedented era of peace in Europe.

So what warnings does that history hold about wars we should and shouldn't fight, and more important: What lessons does it offer about how to keep the peace?

In a saner world, President Trump would pay a visit on April 6 to the site in Flanders fields, where the Battle of Passchendaele was fought in 1917 in an effort to take a bulge of territory around the medieval cloth town of Ypres. In a saner world, the visit might moderate his take on nationalism and on America First.

The British WWI poet Sigfried Sassoon wrote, "I died in hell - (They called it Passchendaele)." This offensive has become synonymous with the insanity of useless battles and callous commanders.

As I stood on a calm hill, next to a Canadian memorial for the fallen, it was hard for me to grasp the horrors that occurred on the flat farmlands below.

The British field marshal, Sir Douglas Haig, believed he could break through German lines and reach the sea in three weeks, but he launched the battle in July 1917 without enough preparation. British shelling had destroyed lowland drainage systems and churned clay soil. Heavy rains made the quagmire even more deadly, drowning horses and men. By early November when the campaign ended, the allies had suffered roughly 325,000 casualties (while the Germans lost about 260,000) for a gain of five miles.

Nearby, outside the village of Saint-Julien, I stood beneath the haunting 35-foot-high statue of the Brooding Soldier, holding his upside-down sword like a cross to honor the dead. Near that site, the Germans launched the first (chlorine) gas attack of the war in 1915, killing 2,000 Canadian troops. The gas melted soldiers' lungs and they drowned in their own fluids or suffered blinding or dreadful burns. Unspent gas mixed with pools of water, turning the water to acid, which devoured many soldiers.

Unbelievably, chlorine gas is still being used against civilians by Syrian President Bashar al-Assad, with full knowledge of the Russians, and in defiance of a U.S.-Russian deal that banned other forms of chemical weapons. One hundred years after World War I, the world can't or won't punish the use of poison gas.

At the British Tyne Cot cemetery more than 11,000 stark white crosses march in rows behind a gate that lists 37,000 missing soldiers. Forty-four thousand German war dead are buried nearby at the Langemark cemetery, many in mass graves.

But these grim memorials are given deeper meaning by what happened after two world wars ravaged Europe. These wars proved - unless one was blind - that aggressive nationalism led to carnage; Franco-German hatred proved especially suicidal.

So, after WWII, the European common market, and then the European Union, were designed to embed Germany within a common European home that included France and Britain, and to bury the virulent nationalism that had devastated the continent. Once the Berlin Wall fell, central and Eastern European countries were tied into this fabric.

Whatever the EU's problems, this "European project" has kept the peace on the continent for the last seven decades.

Yet, even as the WWI centenary goes on, historical memory seems to have faded in much of Europe - and in the White House. Britain has Brexited the European Union. French presidential candidate Marine Le Pen says she wants to "destroy" the EU, if she wins the April-May election, which she has a chance of doing. Le Pen is also whipping up hostility among her followers toward Germany.

Meantime, a host of nationalist political parties within Europe, some, including Le Pen's National Front (with its historical roots in neofascism), look to Russia's uber nationalist, Vladimir Putin, as their role model. They praise a Russian leader who openly aspires to destroying European unity and has invaded his neighbors in a push for territorial expansion.

Which brings us to the White House, where President Trump's chief strategist, Steve Bannon, has said: "I think strong nationalist movements in countries make strong neighbors and that is really the building blocks that built Western Europe and the United States." He and his boss extol Putin and dump on the Europeanist Angela Merkel. He praises the pre-1914 era as the moment when "capitalism was at its highest flower."

Has Bannon's brain blanked out Europe's 1914 descent into a half-century of destruction? Or the virulent nationalism that caused WWI and WWII?

Can he (and Trump) not discern the distinction between patriotism and the nationalism of trade wars and territorial aggression that leads to conflicts? One need only visit Flanders fields to understand why a return to the nationalism of 1914, or the 1930s, would spell disaster. It's bad enough that Le Pen and Putin ignore the history that cost Europe so dearly, but it's shameful for the White House to do the same.