What if Hitler had invaded Britain?

For more than 1000 years, the tribes of Europe have stared into the gunmetal-grey chop of the English Channel and thought of conquest.

‘‘We have six centuries of insults to avenge,’’ Napoleon said. I was just there, on the same spring week when the great bedraggled scraps of the French and British armies were cornered for slaughter by the Nazi war machine 77 years ago.

Perhaps many people did not know, as Donald Trump says about obvious things that he just learned about, that the miracle of the Dunkirk evacuation continues to prompt one of the great historical ‘‘what-ifs’’ of all time.

Alternative history is all the rage now. One television series imagines a United States under German and Japanese rule

Thanks to the film Dunkirk, minds otherwise gone soft by the heartless current governing policies of the United States can turn to a day when bigger minds guided Western democracies. Aside from slighting the French, the film is an energy drink of speculative fuel.

Between May 26 and June 4, 1940, the British were able to evacuate more than 330,000 allied troops from the French beach at Dunkirk, aided considerably by a flotilla of fishing boats, pleasure craft and other small vessels. The troops had been routed.

By the close of summer of 1940, most of Western Europe was under German control or that of puppet states. Would England fall as well and usher in a Nazi empire, with its horrific Jewish genocide, that might have lasted well beyond the war’s end in 1945?

The first big question is why didn’t the Germans finish off the stranded forces of good? Shockingly, one day before the evacuation, the Blitzkrieg took a break. Historians say German troops needed rest and wanted to consolidate their forces for a final push of the allies into the sea.

And the French, still ridiculed for letting Paris fall in barely a month’s time, heroically held back the Germans for a few days at the Dunkirk perimeter. They may have saved upward of 100,000 lives with their rearguard action.

Later, a German commander called the order to halt one of the biggest blunders of the war. Most of those rescued soldiers would live to fight another day, many landing at Normandy when the Allies returned to retake the Continent four years later.

The fiasco at Dunkirk became a psychic triumph in England. But that brings up the second big question: At the height of their power, why didn’t the Germans cross the channel and march to London?

Here we have the prospect of the Nazis developing an atomic bomb. Imagine the Germans controlling all of Europe to the Ural Mountains, giving the most evil of men enough time to develop the most lethal of weapons.

As Winston Churchill said, it was not just Britain at stake, but lands under imperial control, including the Middle East oil fields. The United States, which had its own appeasement movement at odds with Franklin Roosevelt, was a long way from entering the war.

Hitler’s generals drew up plans for an invasion of Britain, Operation Sea Lion. Before there could be an invasion, the Luftwaffe would have to destroy the Royal Air Force.

England held what became its ‘‘finest hour’’ – in Churchill’s bestknown phrase – because the Royal Air Force bested Hitler’s air force in the Battle of Britain.

Even if England were occupied, it’s fair to think a vigorous resistance movement would have forced Germany to keep a huge reserve of troops in place to hold down the island kingdom. And Hitler, of course, had other things in mind – the invasion of the Soviet Union.

Walking along the sand of the English Channel, you can’t help seeing those shivering boys at Dunkirk, part of the World War II generation that will soon be gone entirely. Before they disappear into the churn of history, we owe them another deep thanks for the speculation that can remain just that.

New York Times