This writer was 11 years old when the shocking news came on June 25, 1950, that North Korean armies had crossed the DMZ.

Within days, Seoul had fallen. Routed U.S. and Republic of Korea troops were retreating toward an enclave in the southeast corner of the peninsula that came to be known as the Pusan perimeter.

In September came Gen. MacArthur’s masterstroke: the Marine landing at Inchon behind enemy lines, the cut-off and collapse of the North Korean Army, recapture of Seoul and the march to the Yalu.

“Home by Christmas!” we were all saying.

Then came the mass intervention of a million “volunteers” of the People’s Liberation Army that had, in October 1949, won the civil war against our Nationalist Chinese allies. Suddenly, the U.S. Army and Marines were in headlong retreat south. Seoul fell a second time.

There followed a war of attrition, the firing of MacArthur, the repudiation of Harry Truman and his “no-win war,” the election of Ike and, in June 1953, an armistice along the DMZ where the war began.

Fifty-seven years after that armistice, a U.S. carrier task force is steaming toward the Yellow Sea in a show of force after the North fired 80 shells into a South Korean village.

We will stand by our Korean allies, says President Obama. And with our security treaty and 28,000 U.S. troops in South Korea, many on the DMZ, we can do no other. But why, 60 years after the first Korean War, should Americans be the first to die in a second Korean War?

Unlike 1950, South Korea is not an impoverished ex-colony of Japan. She is the largest of all the “Asian tigers,” a nation with twice the population and 40 times the economy of the North.

Seoul just hosted the G-20. And there is no Maoist China or Stalinist Soviet Union equipping Pyongyang’s armies. The planes, guns, tanks and ships of the South are far superior in quality.

Why, then, are we still in South Korea? Why is this quarrel our quarrel? Why is this war, should it come, America’s war?

High among the reasons we fought in Korea was Japan, then a nation rising from the ashes after half its cities had been reduced to rubble. But, for 50 years now, Japan has had the second largest economy and is among the most advanced nations on earth.

Why cannot Japan defend herself? Why does this remain our responsibility, 65 years after MacArthur took the surrender in Tokyo Bay?

The Soviet Empire, against which we defended Japan, no longer exists, nor does the Soviet Union. Russia holds the southern Kurils, taken as spoils from World War II, but represents no threat. Indeed, Tokyo is helping develop Russia’s resources in Siberia.

Why, when the Cold War has been over for 20 years, do all these Cold War alliances still exist?

Obama has just returned from a Lisbon summit of NATO, an alliance formed in 1949 to defend Western Europe from Soviet tank armies on the other side of the Iron Curtain that threatened to roll to the Channel. Today, that Red Army no longer exists, the captive nations are free, and Russia’s president was in Lisbon as an honored guest of NATO.

Yet we still have tens of thousands of U.S. troops in the same bases they were in when Gen. Eisenhower became supreme allied commander more than 60 years ago.

Across Europe, our NATO allies are slashing defense to maintain social safety nets. But Uncle Sam, he soldiers on.

We borrow from Europe to defend Europe. We borrow from Japan and China to defend Japan from China. We borrow from the Gulf Arabs to defend the Gulf Arabs.

To broker peace in Palestine, Obama began his presidency with a demand that Israel halt all new construction of settlements in East Jerusalem and the West Bank.

Today, as his price for a one-time-only 90-day freeze on new construction on the West Bank, but not East Jerusalem, “Bibi” Netanyahu is demanding 20 F-35 strike fighters, a U.S. commitment to a Security Council veto of any Palestinian declaration of independence, and assurances the U.S. will support a permanent Israeli presence on the Jordan river. And the Israelis want it all in writing.

This, from a client state upon which we have lavished a hundred billion dollars in military aid and defended diplomatically for decades.

How to explain why America behaves as she does?

From 1941 to 1989, she played a great heroic role as defender of freedom, sacrificing and serving mankind, a role of which we can be forever proud. But having won that epochal struggle against the evil empire, we found ourselves in a world for which we were unprepared. Now, like an aging athlete, we keep trying to relive the glory days when all the world looked with awe upon us.

We can’t let go, because we don’t know what else to do. We live in yesterday — and our rivals look to tomorrow.