The short answer is that the politicians learned only one lesson from the Vietnam war, instead of the many they should have learned. That lesson was that if you are going to be suckered into a war by the military-industrial complex, avoid conscription.

Conscription brings those facing conscription on to the streets along with their mothers and sisters and other family members. That can result in electoral pain.

But with no conscription, opposition to the war in Afghanistan has been muted, even though majorities in both the US and Australia would support withdrawal. But it is not a vote-changing issue, in the way that Vietnam was.

That said, the only realistic choice for the US in Afghanistan is to do what the British did in 1842 and the Soviet Union did in 1989 - just withdraw.

But no. The US is going through the same doomed exercise it did in Vietnam - propping up a local government with dubious democratic credentials while negotiating a peace with the other side, knowing it will not last a day after withdrawal. Only 20 per cent turned out for the presidential election and the winner got 50.6 per cent of the vote - not a big endorsement.

Whatever the US does, Australia should withdraw its remaining 1550 troops. We have no reason to be there. We never had any reason to be there. The Afghan War (both Australia's and the US's longest wars) along with the illegal invasion of Iraq have made both countries more vulnerable to terrorism, not less.

And the cost in blood and treasure has been immense. For Australia is has been 41 dead; 261 wounded; $9.3 billion in direct spending; untold millions in treatment for PTSD and other conditions for many of the 26,000 who have served there; and the costs of the inspector-general of the ADF's inquiry into alleged war crimes committed by Australians.

For the US, it has been 2400 dead; 20,000 wounded; $2 trillion on direct and indirect costs so far and untold millions in future treatment.

For Afghanistan, the civilian losses have not been fully tallied, but the economic and human cost has been crippling.

And for what? The evil Taliban are biding their time to sign a sham peace agreement and take over the country again with their hellish treatment of women, strict application of radical Islam and intolerance of any dissent.

The US has not learned the two main lessons from Vietnam. First, that the proposition you can send troops to a foreign country in order to create a stable democratic society is delusional. Second, that the presence of US troops will at most be seen as welcome for a very short time before resentment sets in and violence is directed at those troops from anywhere and everywhere.

If the US had not sent massive amounts of arms to Israel since the 1960s and troops and weapons to Saudi Arabia - seen as an enemy of Shi-ite Muslims - since the 1950s and troops and weapons to other regimes in the Middle East or endlessly meddled on Israel's side in the Palestinian conflict, anti-US sentiment would not have built up. It would not have built up to the extent that terrorist groups would make the US a prime target.

But the military-industrial complex has become so politically powerful and pulsatingly profitable since Republican President Dwight Eisenhower warned against it in 1961, that US foreign policy has invariably turned to expensive military intervention.

President Trump is half right when he rails against the huge cost of US military presence abroad and the US's failure to "win". He was right when he said the US should just withdraw from Afghanistan, but he has always been dissuaded by the military and industry people who surround him.

They then appeal to Trump's base business instincts and urge him to demand US allies pay more for their own defence. In other words, let's have more military spending.

The better solution to Trump's concerns that allies are not pulling their weight and that the burden on the US is too high, is not to demand more spending by the allies, but less spending by the US. It is absurd and unnecessary that the US spends more on its military than the next seven biggest spenders combined.

President Obama, too, promised to get out of Afghanistan. But the military and industry people who surrounded him persuaded him to do the opposite. He ordered the "surge" and by 2011, 100,000 US troops were in Afghanistan. It made no difference. The US was on the cusp of another Vietnam but - in the absence of conscription - without the public outcry.

The US now has about 80,000 troops in the Middle East. All they do is stir up resentment. They do exactly the opposite of their alleged purpose. They make the US a target and less safe from terrorism.

The suppliers to the military, whether direct or indirect, supply campaign funding to critical members of Congress, especially key committee chairs and members. It ensures military spending rarely gets cut and that there is always money available to new incursions.

The only exception has been when the Tea Party-inspired budget limits applied across the board, including to the military.

But by and large the insidious influence of military suppliers in Washington explain the disjoin between majority opinion which says the Afghan mission has been unsuccessful and the US should withdraw, on one hand, and what the Administration does on the other.

As long as there is a war, an incursion or at least a big military presence in a few parts of the world, the military-industrial complex is happy. It does not matter whether it is a "fight against communism" or a "war on terror"; as long as it a big, expensive, profitable exercise for them. And too bad about the economic and human cost for the US soldiers and public, its allies, or the people being attacked.

How much better if the US and its democratic allies led by example - by bolstering their own democratic credentials rather than attempting the impossible task of generating democracy elsewhere through the barrel of a gun?

The best way out of the Afghan mess is to just leave.