A massive truck bomb exploded in what was believed to be a secure diplomatic area of Kabul, Afghanistan on June 1, killing at least 90 people, mostly civilians, and wounding more than 450. (Screen grab from CNN)

"We are there and we are committed" was the regular retort of Secretary of State Dean Rusk during the war in Vietnam.

Whatever you may think of our decision to go in, Rusk was saying, if we walk away, the United States loses the first war in its history, with all that means for Southeast Asia and America's position in the world.

We face a similar moment of decision.

Wednesday, a truck bomb exploded near the diplomatic quarter of Kabul, killing 90 and wounding 460. So terrible was the atrocity that the Taliban denied complicity. It is believed to have been the work of the Haqqani network.

This "horrific and shameful attack demonstrates these terrorists' compete disregard for human life and their nihilistic opposition to the dream of a peaceful future for Afghanistan," said Hugo Llordens, a U.S. diplomat in Kabul.

The message the truck bombers sent to the Afghan people? Not even in the heart of this capital can your government keep civilian workers and its own employees safe.

Message to America: After investing hundreds of billions and 2,000 U.S. lives in the 15 years since 9/11, we are further from victory than we have ever been.

President Obama, believing Afghanistan was the right war, and Iraq the wrong war, ramped up the U.S. presence in 2011 to 100,000 troops. His plan: Cripple the Taliban, train the Afghan army and security forces, stabilize the government, and withdraw American forces by the end of his second term.

Obama fell short, leaving President Trump with 8,500 U.S. troops in Afghanistan, and Kabul's control more tenuous than ever. The Taliban hold more territory and are active in more provinces than they have been since being driven from power in 2001. And Afghan forces are suffering casualties at the highest rate of the war.

Stated starkly, the war in Afghanistan is slowly being lost.

Indeed, Trump has inherited what seems to be an unwinnable war, if he is not prepared to send a new U.S. army to block the Taliban from taking power. And it is hard to believe that the American people would approve of any large reintroduction of U.S. forces.

The U.S. commander there, Gen. John Nicholson, has requested at least 3,000 more U.S. troops to train the Afghan army and stabilize the country while seeking a negotiated end to the war.

Trump's conundrum: 3,000 or 5,000 more U.S. troops can at best help the Afghan security forces sustain the present stalemate.

But if we could not defeat the Taliban with 100,000 U.S. troops in country in 2011, we are not going to defeat a stronger Taliban with a U.S. force one-seventh of that size. And if a guerrilla army does not lose, it wins.

Yet it is hard to see how Trump can refuse to send more troops. If he says we have invested enough blood and treasure, the handwriting will be on the wall. Reports that both Russia and Iran are already talking to the Taliban suggest that they see a Taliban takeover as inevitable.

Should Trump announce any timetable for withdrawal, it would send shock waves through the Afghan government, army and society.

Any awareness that their great superpower ally was departing, now or soon, or refusing to invest more after 15 years, would be a psychological blow from which President Ashraf Ghani's government might not recover.

What would a Taliban victory mean?

The Afghan people, especially those who cast their lot with us, could undergo something like what befell the South Vietnamese and Cambodians in 1975. It would be a defeat for us almost as far-reaching as was the defeat for the Soviet Union, when the Red Army was forced to pull out after a decade of war in the 1980s.

For the USSR, that Afghan defeat proved a near-fatal blow.

And if we pulled up stakes and departed, the exodus from Afghanistan would be huge and we would face a moral crisis of how many refugees we would accept, and how many we would leave behind to their fate.

Fifteen years ago, some of us argued that an attempt to remake Afghanistan and Iraq in our image was utopian folly, almost certain, given the history and culture of the entire region, to fail.

Yet we plunged in.

In 2001, it was Afghanistan. In 2003, we invaded and occupied Iraq. Then we attacked Libya and ousted Gadhafi. Then we intervened in Syria. Then we backed the Saudi war to crush the Houthi rebels in Yemen.

Given the trillions sunk and lost, and the hundreds of thousands, if not millions, dead, how have we benefited ourselves, or these peoples?

As Rusk said, "We are there and we are committed."

And the inevitable departure of the United States from the Middle East, which is coming, just as the British, French and Soviet empires had to depart, will likely do lasting damage to the American soul.

Patrick J. Buchanan is the author of a new book, "Nixon's White House Wars: The Battles That Made and Broke a President and Divided America Forever."