After America’s ignominious defeat and hurried departure from Vietnam in 1973 — when the world’s richest and mightiest nation was humbled by the stolid determination of ill-equipped, ideologically inspired peasants — it was generally assumed the United States would not wage war again until the lessons of the Viet Cong victory were taken to heart.

When Soviet forces hastily retreated with a bloody nose from their nine-year occupation of Afghanistan in 1989, similar lessons were suggested about the impossibility of militarily holding a country with a universally hostile population.

In his stealth occupation of Crimea and eastern Ukraine, President Vladimir Putin of Russia appears to have learned the lessons of both Vietnam and Afghanistan.

Successive U.S. presidents, however, seem to have failed to understand how military strategy was forever changed by what happened in those two chastening conflicts. Rather, they have gone on to repeat their predecessors’ mistakes.

That’s not all. The fleet of U.S. stealth bombers ($810 million each) and the fleet of nuclear submarines ($8.2 billion each) armed with Trident nuclear missiles ($31 million each) are of little use against Russian intelligence agents provocateurs disguised as Ukrainian protesters arriving by civilian airliner.

Neither the United States nor the European Union nor the North Atlantic Treaty Organization has a military solution for the creeping warfare that Putin is putting into chilling effect.

A question should be asked: Has the Pentagon’s decades-long high-tech spending spree — the delight of the domestic war-materials industries — left the West’s defense fit for a purpose? The obvious answer — no! — plays into the hands of American libertarians who are now joining with liberals to demand substantial defense cuts.

But back to Vietnam, a place that still haunts the U.S. military. Thinking they were better than the colonial French generals, who hastily evacuated the country after their humiliating defeat by the Viet Minh at Điên Biên Phu, U.S. commanders believed they could halt the communist insurgency from the north by sheer weight of force. They applied carpet bombing and an overwhelmingly better-equipped and better-trained conscript army.

Our defeat in Vietnam, after four presidents had taken turns trying to win the war by escalating the violence, shocked the nation. David had beaten Goliath, and small arms had trounced big battalions.

For 16 years, the United States took stock. If its military strength counted for so little in small-scale conflicts it was now being asked to fix, what was the way forward?

There was no obvious answer before the next time Washington felt itself obliged to intervene in a major conflict. President George H.W. Bush, with the bellicose British Prime Minister Margaret Thatcher snapping at his heels, decided a Western coalition must recapture Kuwait from Iraqi President Saddam Hussein in the Gulf War of 1991.

The United States returned to the battlefield — a barren stretch of sand highly suitable for mounting what could be the last of the old-school massed tank battles — and annihilated Hussein’s wretchedly ill-equipped forces. Bush declined to push on to Baghdad and topple Hussein, a wise decision informed not only by the horrors of Vietnam but by the humiliation of the Soviets in Afghanistan.

The fact that some of the greatest military forces the world has ever seen — from Alexander the Great’s army to the soldiers of the mighty British Empire at its height — had failed to subdue the Afghans might have given the old men in the Kremlin pause for thought. Like the Pentagon, however, they pressed on in the belief that a few ill-armed mujahedeen would soon succumb to the overwhelming superiority of sophisticated tanks and planes.

It didn’t turn out that way. As in Vietnam, “asymmetrical warfare” — the unequal weight of forces on either side — meant the Afghan nationalists confronted the 100,000-strong invading Soviet army with guerrilla tactics. By the time the Russians made their speedy exit, in February 1989, 15,000 of the invading army had been killed.

The prospect of U.S. forces being similarly slaughtered while trying to occupy an anarchic post-Hussein Iraq gave Bush pause. Instead he opted for crippling the country with an economic sanctions regime, including an oil embargo, and pinning down Hussein’s forces with a no-fly zone — a containment policy followed by his successor, President Bill Clinton.

The terrorist attacks of September 11, 2001 changed everything. Not least, they encouraged an absurd sense of hubris in the White House under President George W. Bush and Vice President Dick Cheney. Al Qaeda had extended the guerrilla logic of the Viet Cong and the mujahedeen by using terrorism against soft U.S. targets, of which the hijacking of airliners proved the most devastatingly effective.

For the rest of the world, which since the arrival of terrorism as a means of warfare had become accustomed to searches before boarding a plane, the lack of security at U.S. airports had seemed careless if not downright neglectful.

Just as careless was the indignant response of the Bush administration to the September 11 attacks: the invasion of, of all places, Afghanistan, that had proved the graveyard of the Soviet Union, and the invasion and occupation of Iraq, under the pretext, unbacked by any convincing intelligence subsequently produced, that Hussein had weapons of mass destruction he was about to use against the West.

President Barack Obama defeated his principal Democratic rival, Hillary Clinton — and held off the Republican hawk Senator John McCain (R-Ariz.) in the general election – in part because he had vocally opposed the war in Iraq and had expressed profound reservations about the conflict in Afghanistan.

Obama campaigned on a promise to bring both wars to an end. His approach to conflicts on his watch has been principally to dodge them. In Libya, he allowed his European allies to take the lead. When Syrian President Bashar al-Assad’s government forces used chemical weapons against rebels, Obama hid behind Congress. These chemical attacks continue.

Now the United States and its wavering ally the European Union are faced with what to do about Russia’s surreptitious expansionism westward. Understanding the lessons of Vietnam, Afghanistan and Iraq, Putin has invaded Ukraine in disguise — first annexing Crimea and now, through not-so-covert forces, destabilizing largely Russian-speaking eastern Ukraine to provide himself with an open invitation to invade with tanks if and when he thinks fit.

The West’s response? Passport bans, economic sanctions against a small number of Russian friends of Putin and the suspension, not even expulsion, of Russia from the Group of 8.

There is no military option. U.S. taxpayers hand over $682 billion to the Pentagon each year, and yet there is no military option. Instead, Secretary of State John Kerry complains that Putin isn’t playing fair by adopting a 19th century approach to a 21st century problem.

It may be that Ukraine is already lost to Putin. It may be Putin will annex other former Soviet republics, including Georgia, Kazakhstan, Kyrgyzstan, Moldova, Turkmenistan, Tajikistan, Uzbekistan, Azerbaijan and Belarus. (Putin is unlikely to grab the Baltic states of Estonia, Latvia and Lithuania since they are all members of NATO.)

How will the West respond if he keeps invading? That is anyone’s guess. Most likely, though, each invasion will be met with anguished concern but no Western military response.

So it is all down to sanctions. Will they work? Ask Cuba. Or North Korea. Or Zimbabwe. Or Iran.

PHOTO (TOP): Military personnel, believed to be Russian servicemen, march outside the territory of a Ukrainian military unit in the village of Perevalnoye outside Simferopol, March 4, 2014. REUTERS/David Mdzinarishvili

PHOTO (INSERT 1): Russian President Vladimir Putin (front R) meets with newly appointed high-ranking military officers during a ceremony in the Kremlin in Moscow, March 28, 2014. REUTERS/Alexei Druzhinin/RIA Novosti/Kremlin

PHOTO (INSERT 2): 173rd Airborne Brigade under fire on Hill 823 in Vietnam, November 1967. REUTERS/U.S. Army

PHOTO (INSERT 3): A wounded soldier is carried to a helicopter in the Ia Drang Valley, in Vietnam, November 1965. REUTERS/U.S. Army

PHOTO (INSERT 4): Afghan guerrillas known as Mujahideen, have lunch near a Soviet helicolpter shot down during a rebel attack in Nuristan, Afghanistan, date unknown. REUTERS/John O’Brien

PHOTO (INSERT 5): Soviet officers and soldiers leave their base outside Afghan capital Kabul as they return to the Soviet Union, March 1988. REUTERS/Sergei Karpukhin

PHOTO (INSERT 6): Military personnel, believed to be Russian servicemen, march outside the territory of a Ukrainian military unit in the village of Perevalnoye outside Simferopol, March 4, 2014. REUTERS/David Mdzinarishvili