A thin fragment of moon stood watch that Christmas Eve as the president of the United States and the prime minister of Great Britain came out onto the South Portico of the White House. They were there to light the national Christmas tree — and to speak a holiday greeting to an uncertain world.

Two and a half weeks before — and 70 years ago today — the Japanese had devastated the American Pacific fleet at Pearl Harbor in Hawaii. A nation that had endured 12 grinding years of economic catastrophe was now plunged into a maelstrom of worldwide war. It was Christmas in time of turmoil, a season of brotherhood and peace under the shadow of genocide and war — and it fell to these two men to help the nation and the world make sense of that.

These times, thank God, are not like those. Though the nation finds itself mired in the worst economic disaster since the Depression of the 1930s, though it fights multiple wars, though terrorism is an ever present menace, America faces no existential danger, no threat to its very survival, as it did 70 Christmases ago.

Which is not to say these are easy times. If America’s continued existence is not in doubt, there is, nevertheless, fear of the shape that existence will take. America seems diminished by her woes. Besides the wars, besides the economy, besides the terror, there are the schools, which are not educating; the infrastructure, which is cracking; the debt, which is ballooning; the anger, which is boiling; the divisions, which are widening.

And there is the hope, which is fading. In May, a Gallup poll found optimism for the future has fallen to record lows. Just 44 percent of Americans believe today’s youth will have better lives than their parents.

Is there anything more redolent of America than optimism? When it is lost, something essential to the nation’s character is, too.

No, these times are not like those. In that America, people girded for sacrifice and sang new Christmas carols that rang bittersweet with uncertainty. “I’ll be home for Christmas,” went one, “if only in my dreams.” Another said, “Someday soon we all will be together — if the fates allow.”

In our America, people pepper spray or trample one another to get deals on video games and DVD players.

But both Americas were challenged, both shaken, both reached a year’s end in the shadow of a future that loomed foreboding and grim. As we wait to see how this America will respond, it is good to recall how that one did.

“Against enemies who preach the principles of hate and practice them,” said Roosevelt, “we set our faith in human love and in God’s care for us and all men everywhere.”

“Let the children have their night of fun and laughter,” said Churchill. “Let the gifts of Father Christmas delight their play. Let us grownups share to the full in their unstinted pleasures before we turn again to the stern task and the formidable years that lie before us, resolved that, by our sacrifice and daring, these same children shall not be robbed of their inheritance or denied their right to live in a free and decent world.”

Radio carried their words around the globe. Thousands more were there in person, standing beneath the shining tree. Under a rind of moon in a time of war, they gazed up, and were bathed in Christmas light.