In this world, it is often dangerous to be an enemy of the United States, said Henry Kissinger in 1968, but to be a friend is fatal.
The South Vietnamese would come to appreciate the insight.
So it is today with Aleppo, where savage reprisals against U.S.-backed rebels are taking place in that hellhole of human rights.
Yet, again, the wrong lessons are being drawn from the disaster.
According to The Washington Post, the bloodbath is a result of a U.S. failure to intervene more decisively in Syria's civil war: "Aleppo represents a meltdown of the West's moral and political will -- and ... a collapse of U.S. leadership.
"By refusing to intervene against the Assad regime's atrocities, or even to enforce the 'red line' he declared on the use of chemical weapons, President Obama created a vacuum that was filled by Vladimir Putin and Iran's Revolutionary Guard."
But the blunder was not in staying out of Syria's civil war, but in going in. Aleppo is a bloodbath born of interventionism.
On Aug. 18, 2011, President Obama said, "For the sake of the Syrian people the time has come for President Assad to step aside." Western leaders echoed the Obama -- "Assad must go!"
Assad, however, declined to go, and crushed an Arab Spring uprising of the kind that had ousted Hosni Mubarak in Cairo. When the U.S. began to fund and train rebels to overthrow him, Assad rallied his troops and began bringing in allies -- Hezbollah, Iran and Russia.
It was with their indispensable assistance that he recaptured Aleppo in the decisive battle of the war. And now America has lost credibility all over the Arab and Muslim world.
How did this debacle come about?
First, in calling for the overthrow of Bashar Assad, who had not attacked or threatened us, we acted not in our national interests, but out of democratist ideology. Assad is a dictator. Dictators are bad. So Assad must go.
Yet we had no idea who would replace him.
It soon became clear that Assad's most formidable enemies, and probable successors, would be the al-Nusra Front, the Syrian branch of al-Qaida, or ISIS, then carrying out grisly executions in their base camp in Raqqa.
U.S. policy became to back the "good" rebels in Aleppo, bomb the "bad" rebels in Raqqa and demand that Assad depart. An absurd policy.
Nor had the American people been consulted.
After a decade of wars in Iraq and Afghanistan, they saw no U.S. vital interests at risk in who ruled Damascus, so long as it was not the terrorists of ISIS or al-Qaida.
Then came Obama's "red line" warning: The U.S. would take military action if chemical weapons were used in Syria's civil war.
What undercut this ultimatum was that Congress had never authorized the president to take military action against Syria, and the American people wanted to stay out of Syria's civil war.
When Assad allegedly used chemical weapons and Obama threatened air strikes, the nation rose as one to demand that Congress keep us out of the war. Secretary of State John Kerry was reduced to assuring us that any U.S. strike would be "unbelievably small."
By 2015, as Assad army's seemed to be breaking, Vladimir Putin boldly stepped in with air power, alongside Hezbollah and Iran. Why? Because all have vital interests in preserving the Assad regime.
Bashar Assad is Russia's ally and provides Putin with his sole naval base in the Med. Assad's regime is the source of Hezbollah's resupply and weapons to deter, and, if necessary, fight Israel.
To Iran, Assad is an ally against Saudi Arabia and the Sunni awakening and a crucial link in the Shiite Crescent that extends from Tehran to Baghdad to Damascus to Beirut.
All have greater stakes in this civil war than do we, and have been willing to invest more time, blood and treasure. Thus they have, so far, prevailed.
The lessons for Trump from the Aleppo disaster?
Do not even consider getting into a new Middle East war -- unless Congress votes to authorize it, the American people are united behind it, vital U.S. interests are clearly imperiled, and we know how the war ends and when we can come home.
For wars have a habit of destroying presidencies.
Korea broke Truman. Vietnam broke Lyndon Johnson. Iraq broke the Republican Congress in 2006 and gave us Obama in 2008.
And the Iran war now being talked up in the think tanks and on the op-ed pages would be the end of the Trump presidency.
Before starting such a war, Donald Trump might call in Bob Gates and ask him what he meant at West Point in February 2011 when he told the cadets:
"Any future defense secretary who advises the president to again send a big American land army into Asia or into the Middle East or Africa should 'have his head examined,' as General MacArthur so delicately put it."