Picture this. You are a 20-something-year-old Australian soldier in Afghanistan.
Your job is to help train soldiers of the Afghan National Army so they can take over responsibility for security in Oruzgan province.
You've been in the country several months for the peak of the Taliban fighting season.
It's been tough: patrols frequently encounter improvised bombs and insurgents regularly take pot-shots from the dense vegetation.
Despite everything, you feel you're doing some good. The ANA seem to be making progress and patrol on their own more often.
Then one day something goes horribly wrong. An Afghan soldier takes his assault rifle and starts shooting at a group of Australians inside your shared patrol base.
Mates lie dead, others are wounded and the perpetrator has fled.
When the shock passes and the dead and wounded have been airlifted out, what do you think about the Afghan soldiers you work with? Do you still trust them?
The issue of trust is at the heart of the murder of three Australian soldiers and wounding of two others inside a joint patrol base.
Our strategy in Afghanistan rests on the notion that our troops will build and maintain positive relationships with the Afghan soldiers they are training.
The two groups must trust each other because there is ample mistrust in Afghanistan already - mistrust of President Hamid Karzai's Government, mistrust of local officials, mistrust of other tribes, mistrust of the coalition. It is vital the Australian and ANA soldiers can at least rely on each other.
Would you turn your back on an Afghan soldier during a gunfight if you suspected he might be thinking of shooting you, not the enemy?
Without a framework of mutual respect and trust, our strategy cannot succeed. We cannot simply wish into existence a competent, independent Afghan fighting force.
It has taken years of difficult, unrelenting, often bloody work by Australians for the ANA to even begin to look and act like a real army.
A lot more hard work and even more trust will be required before we withdraw most of our combat troops in 2014.
It is a measure of the discipline and resilience of our troops that there has never been a disproportionate response to such outrages.
The challenge for our commanders now is to find a way to get training and mentoring back on track.
Not much more can be done to physically protect our soldiers from such attacks.
Better screening of ANA recruits has begun but there is little likelihood this will be effective because most "green on blue" attacks are by disgruntled Afghan soldiers.
Some extremists do join the security forces but are not yet a major threat.
Cultural affront might tip an Afghan soldier to turn his gun on Westerners. Or he might just have been paid or coerced to do it.
Whatever the cause, there will probably be more attacks by Afghan soldiers on Australians.
The risks to our troops will increase, not decrease, as we adjust our role in Oruzgan province.
As always, getting into a war is easy. The hard part is leaving. And the really hard job is getting on with the mission alongside Afghan troops who might shoot you.
And this all comes down to that 20-something Australian soldier and his mates.
John Cantwell was commander of Australian forces in Afghanistan and Iraq from 2010-11. He retired from the army this year with the rank of Major-General. His book Exits Wounds (Melbourne University Publications) will be published in September.