Of all the mendacious nonsense that pours out of politicians' mouths, David Cameron's claim that British combat troops will be coming home from Afghanistan with their "mission accomplished" is in a class all of its own. It's almost as if, by echoing George Bush's infamous claim of victory in Iraq in May 2003 just as the real war was beginning, the British prime minister is deliberately courting ridicule.

But British, American and other Nato troops have been so long in Afghanistan – twice as long as the second world war – that perhaps their leaders have forgotten what the original mission actually was. In fact, it began as a war to destroy al-Qaida, crush the Taliban and capture or kill their leaders, Osama bin Laden and Mullah Omar.

That quickly morphed into a supposed campaign for democracy and women's rights, a war to protect our cities from terror attacks, to eradicate opium production and bring security and good governance from Helmand to Kandahar. With the exception of the assassination of Bin Laden – carried out 10 years later in another country – not one of those goals has been achieved.

Instead, al-Qaida has mushroomed and spread throughout the Arab and Muslim world, engulfing first Iraq and now Syria. Far from protecting our streets from attacks, the war has repeatedly been cited as a justification for those carrying them out – most recently by Michael Adebolajo, who killed the Afghan war veteran Lee Rigby on the streets of London in May.

The Taliban is long resurgent, mounting 6,600 attacks between May and October this year and negotiating for a return to power. Mullah Omar remains at liberty. Afghan opium production is at a record high and now accounts for 90% of the world's supply. Less than half the country is now "safe for reconstruction", compared with 68% in 2009.

Meanwhile, women's rights are going into reverse, and violence against women is escalating under Nato occupation: 4,000 assaults were documented by Afghan human rights monitors in the first six months of this year, from rape and acid attacks to beatings and mutilation. Elections have been brazenly rigged, as a corrupt regime of warlords and torturers is kept in power by foreign troops, and violence has spilled over into a dangerously destabilised Pakistan.

All this has been at a cost of tens of thousands of Afghan civilian lives, along with those of thousands US, British and other occupation troops. But it's not as if it wasn't foreseen from the start. When the media were hailing victory in Afghanistan 12 years ago, and Tony Blair's triumphalism was echoed across the political establishment, opponents of the invasion predicted it would lead to long-term guerrilla warfare, large-scale Afghan suffering and military failure – and were dismissed by the politicians as "wrong" and "fanciful".

But that is exactly what happened. One study after another has confirmed that British troops massively increased the level of violence after their arrival in Helmand in 2006, and are estimated to have killed 500 civilians in a campaign that has cost between £25bn and £37bn. After four years they had to be rescued by US forces. But none of the political leaders who sent them there has been held accountable for this grim record.

It was the same, but even worse, in Iraq. The occupation was going to be a cakewalk, and British troops were supposed to be past masters at counter-insurgency. Opponents of the invasion again predicted that it would lead to unrelenting resistance until foreign troops were driven out. When it came to it, defeated British troops were forced to leave Basra city under cover of darkness.

But six years later, who has paid the price? One British corporal has been convicted of war crimes and the political elite has shuffled off responsibility for the Iraq catastrophe on to the Chilcot inquiry – which has yet to report nearly three years after it last took evidence. Given the dire lack of coverage and debate about what actually took place, maybe it's not surprising that most British people think fewer than 10,000 died in a war now estimated to have killed 500,000.

But Iraq wasn't the last of the disastrous interventions by the US and Britain. The Libyan war was supposed to be different and acclaimed as a humanitarian triumph. In reality not only did Nato's campaign in support of the Libyan uprising ratchet up the death toll by a factor of perhaps 10, giving air cover to mass ethnic cleansing and indiscriminate killing. Its legacy is a maelstrom of warring militias and separatist rebels threatening to tear the country apart.

Now the west's alternative of intervention-lite in Syria is also spectacularly coming apart. The US, British and French-sponsored armed factions of the Free Syrian Army have been swept aside by jihadist fighters and al-Qaida-linked groups – first spawned by western intelligence during the cold war and dispersed across the region by the invasions of Afghanistan and Iraq.

The wars unleashed or fuelled by the US, Britain and their allies over the past 12 years have been shameful. Far from accomplishing their missions, they brought untold misery, spread terrorism across the world and brought strategic defeat to those who launched them. In the case of Afghanistan all this looks likely to continue, as both the US and Britain plan to keep troops and bases there for years to come.

By any objective reckoning, failures on such a scale should be at the heart of political debate. But instead the political class and the media mostly avert their gaze and wrap themselves in the flag to appease a war-weary public. The first sign that this might be changing was the unprecedented parliamentary vote against an attack on Syria in August. But the democratisation of war and peace needs to go much further. Rather than boasting of calamitous missions, the politicians responsible for them must be held to account.

Twitter: @seumasmilne