If there ever was a moment for citizens of the United States of America to contemplate and appreciate how their nation has affected humankind’s evolution, it is now on its 250th birthday.

Yes, the U.S. is now afflicted with political polarization and cultural angst over an array of specific issues, but it’s been there before and we Americans have somehow survived — even after an extremely bloody civil war.

We continue to be the globe’s most powerful economic and cultural influencer thanks largely to our extraordinarily diverse population, born of our historic willingness to absorb immigrants and refugees.

I, for instance, can trace one thread of my lineage back to a man named George Glenn, one of the hundreds of thousands of Scots Irish who migrated from Ulster during the 18th century and settled in Appalachia. How Glenn’s family evolved from poverty and suppression in Ulster to success in America typifies how the future nation gained enough population and economic strength to break away from Great Britain and declare its independence on July 4, 1776.

I’m proud of that, and all descendants of immigrants should be, whether they came before 1776 or last week. But we should be ashamed that immigration has become a political football, exploited by politicians of both parties.

All of that said, there is one event that encapsulates USA’s unique place in humankind’s history: World War II, when we saved the world during one of its darkest periods.

I am a WWII buff who is repeatedly reminded of just how close the Axis powers or Germany, Japan and Italy came to potentially ending civilization through military conquest. At the time, Americans may have been sympathetic to the plights of nations and territories the Axis had conquered but — seemingly secure behind wide oceanic barriers — were reluctant to get militarily involved.

That changed when the Japanese Imperial Navy attacked our Pacific Fleet in Pearl Harbor on Dec, 7, 1941. We declared war on Japan, Germany declared war on us and the die was cast for the most massive and destructive conflict humankind had ever experienced.

There are multiple reasons why the U.S. and its allies prevailed in 1945, but underlying every aspect is that our nation embraced a moral imperative to rid humankind of those who sought to dominate or enslave everyone else. The U.S. became, in the words of President Franklin Roosevelt, the “arsenal of democracy,” producing copious numbers of airplanes, warships and merchant ships to transport them to the battlefront.

We literally kept Great Britain, the nation from which we sprang in 1776, from starving. We supplied the troops and the weaponry to prevent Nazi Germany from forcing the British to capitulate, and later to drive the Germans out of France, Norway, Belgium, the Netherlands, Austria, Italy and North Africa.

Simultaneously, we almost single-handedly confronted the Empire of Japan, which had conquered much of Southeast Asia and was threatening Australia and India.

It was costly not only in money but in lives. More than 400,000 American soldiers, sailors and airmen were killed and more than 600,000 were wounded. But it needed to be done and we did it.

The war was transformative in many respects. For example, it forced European nations to give up their colonial empires. It allowed the Soviet Union, our wartime ally, to build its own ring of dominated states. With a reformed Western Europe, we held the Soviets in check.

The war hugely impacted California, converting its economy into industrial production and later into technology, igniting a population boom that lasted for 80 years.

We should never forget that the U.S. saved the world when it needed saving. It was our finest hour.

Dan Walters is one of most decorated and widely syndicated columnists in California history, authoring a column four times a week that offers his view and analysis of the state’s political, economic,...