The approach of the 50th anniversary of the end of the war in Indochina has us thinking of January 6, 2021. There came a moment when we were looking at a news photograph of the melee that day at the Capitol when we suddenly clapped a hand over our mouth and stared. For there, among the American flags being waved in protest, was the yellow banner with three red stripes that was the flag of Free Vietnam.

“Until that moment we had managed to miss this strand among the forgotten men and women of the Trump coalition,” we wrote at the time. “Who was flying that flag — and why?” Mr. Trump, after all, was no tribune of Vietnam. Were the flag wavers nursing an inchoate grudge against Congress for having long ago voted to cut off all military aid to our erstwhile ally and abandon them to the long night of communism?

It turns out, we wrote then, that the Vietnamese standard was all over Capitol Hill on January 6 and, according to NBC, is often flown by Vietnamese Americans and not only on January 6. “This flag to me is an anti-Communist flag,” wrote a Vietnamese American woman, Michelle Le, on a Facebook posting quoted by the National Broadcasting Company. NBC reported that she carried a Vietnamese flag on January 6.

“It’s a reminder of my roots and heritage. I had lived through Communism and I know the tyranny and the pain it had inflicted on many families,” Ms. Le was quoted by NBC as saying. What politician, we strive to recall, had embraced her “noble cause,” as President Reagan did in his famous speech to Veterans of Foreign Wars. The question echoes 50 years after the fighting ended in Vietnam.

All the more so, in our view, because Vietnam turned out to be only the first case of America abandoning an ally with whom it had gone to war. In the case of Vietnam, that happened in a series of votes in the Congress in 1974 and 1975. Both houses were controlled by the Democrats, and in the face of pleading by President Ford and Secretary of State Kissinger, voted to curtail military support for Free Vietnam, precipitating a rout.

After Vietnam, of course, came the long train of retreat. It included Iraq, where, during the Obama administration, America abandoned the fight. Then came the bitter abandonment of Afghanistan, where America surrendered, leaving to our enemy, the Taliban, a major base and billions of dollars in equipment. And also abandoned thousands of Afghan allies who had sided with America.

All this has, over the years, taught us to put a premium on how we enter foreign conflicts. It has increased our attention to the role of Congress, which holds the war powers — to raise and support armies, provide and maintain a Navy, declare war, and grant letters of marque and reprisal. We have never lost a war that was properly declared. We’ve come to the view that to quit a properly declared war is unconstitutional.

President Obama made a point during his presidency of repairing America’s failure to acknowledge the honor and, in many cases, heroism of the GIs who appeared in arms in Vietnam. He spoke eloquently on this head and granted several medals of honor for service a generation earlier in Vietnam, including, to mention but one, to Charles Kettles who repeatedly flew under fire to pilot 40 GIs out of an ambush.

Millions on this anniversary will be uplifted by Reagan’s telling of the Vietnamese refugee who was in a dinghy approaching United States Ship Midway in the South China Sea. When the refugee spied a sailor standing lookout on the carrier’s deck, he stood up, waved, and shouted, “Hello, American sailor. Hello, Freedom man.” No wonder so many still find occasion on the coming anniversary to wave the flag of Free Vietnam.