The psychological cost to military observers-turned-witnesses on peacekeeping duty is similar to the damage caused by traditional combat deployments – according to a leading professor in the field.
Exploring standout examples of peacekeeping gone wrong, Tom Sables has spoken to those who had the training and inclination to intervene as atrocities unfolded on the ground, but who were left with their hands tied by the United Nations.
Research suggests long-term complications arise more often from feelings of shame and guilt than from fear.
The unkept peace leading to the Rwanda and Srebrenica genocides added to the sample pool of those who were underequipped, underprepared or unauthorised to take action on their instincts.
Peacekeeping, by its very name, suffers one operational dimension: the force can only go in when there is peace to be kept.
Invited into global hotspots under tenuous conditions and often with restricted rules of engagement, troops on missions gone by have been forced to rely on UN officials who are some distance from the reality on the ground.
Professor Simon Wessely, Director at King's Centre for Military Health Research, says: "What's surprising about the mental health impact of peacekeeping compared to combat missions is how similar they are.
"On the one hand, there are things you can't do that you think you should've done properly, this could be engaging to prevent an atrocity, and the second thing is where you feel you have failed yourself(…) you failed to spot an ambush, you failed to spot an IED.
"It's actually more shame and guilt, than fear, that generates long-term complications."
Prof Wessely adds that Operation Grapple (Britain's mission to support the wider UN peacekeeping in former Yugoslavia) created "a lot more psychiatric problems" than Operation Granby (Britain's mission in the First Gulf War), despite the latter being a conventional warfighting combat operation.
The term 'moral injury', psychological distress following actions that violate an individual's moral code, is separate to PTSD and crystallised during the time of Op Grapple, he explains.
A psychological overlap also exists between peacekeeping trauma and friendly fire cases, he says, as anger felt toward your own side "who accidentally bombed you" can be likened to anger felt towards the United Nations "who gave you rules of engagement that left you completely exposed and unable to act to prevent an atrocity".
The failed siege of Gorazde – the peacekeepers who trained for warfighting
Both peacekeeping and combat missions offer 'quieter' moments, and UN efforts to monitor ceasefires can go years without kinetic aggression.
But weaknesses within the system have been exposed when violence does arrive, and decision-maker Colonel Richard Westley had to work around his rules of engagement as company commander with the Royal Welsh Fusiliers in Gorazde, Bosnia in 1995.
Bosnian Serb aggressors used long-range weapons to target children walking to school from outside an exclusion zone established under a ceasefire and peacekeepers could only fire when fired upon, so Col Westley placed his UN vehicles in the line of fire – technically enabling troops to fire back.
Eventually, peacekeepers made a stand as Serbian forces launched a full-scale assault on Gorazde's Bosnian Muslim community and UN observation posts, killing many while defensive forces rallied.
"It was not our mission, really, but we were forced into that situation," Col Westley says.
He criticises UN efforts to 'control' troops on the ground from HQ in New York which, on one occasion, was unable to organise air support for his peacekeepers under fire as the office was empty on weekends.
Specialists in air support, SAS troops joined the security mission, although it later emerged they had been tasked with monitoring and feeding back reports on the mental health of peacekeepers of all rank – word of the violence they had witnessed reaching back home.
Indiscriminate targeting of children and families mean "you can very easily develop a feeling of helplessness, because you can't protect them – a certain amount of guilt".
Despite receiving recognition from the Queen for his bravery, which saved 10,000 lives, the retired colonel regrets the conditions of his deployment.
Sleepless nights followed although Col Westley opted for encouraging his men to "talk amongst themselves".
"We were just not as well developed in terms of trauma risk management as the forces are now," he says.
"We should have been able and equipped and resourced to do more, but we weren't."
Srebrenica - a 'black page in history'
Moving on from the failed Siege of Gorazde, the 'Butcher of Bosnia' and now convicted war criminal Rakto Mladic's turned their attention to the enclave town of Srebrenica.
The Netherland's Dutchbat III peacekeeper Remko de Bruijne was a sniper in his early 20s at the time and had seen military build-up surrounding the location weeks before Serbian forces made their move.
Explained away as a large exercise despite being at war for years, Mladic's forces essentially walked into Srebrenica – soon ordering UN troops to pile civilians onto buses at a compound.
Although this didn't raise too many alarms for some (separation to isolate fighters from the rest falling under the Geneva convention), Mr de Bruijne had been on a bus himself and knew the fate awaiting many.
He fought to get some Muslims back from a remote farm, but the largest mass-killing in Europe since the Second World War still took place – 8,000 men and boys executed in an atrocity the international war crimes tribunal labelled genocide.
Leaving the army in 2011, Remko was diagnosed with PTSD and received treatment at the Sinai Center, which had typically specialised in concentration camp victims.
Now the founder of a veterans support organisation, he's learned to cope with the "triggers" of his condition, including crowds of people and garbage – having witnessed inhabitants of Srebrenica scavenging for food with supply routes blocked off, even peacekeepers forced to drink from a well.
"I cannot handle the smell of garbage very well. It was an open concentration camp – even the food that we put in the garbage, the people of the Srebrenica town were scavenging."
Rwanda and the commander who saved more than 30,000 lives
The mandates that make peacekeeping missions legal fall under different categories of the UN Charter.
Chapter VI missions seek to keep peace through negotiation, not force – like many of the missions in Bosnia.
Chapter VII means 'peace enforcement' and Lieutenant General Roméo Dallaire, who led the peacekeeping force in Rwanda, says a mandate like this could have made a huge difference to troops on the ground when a 100-day genocide hit in 1994 – death toll estimates ranging from 500,000 to 1,000,000, many buried in mass graves.
The use of child soldiers, rape as an instrument of war and machete killings became commonplace as ethnic Hutu militias brutally targeted Tutsi, Twa and moderate Hutu opposition members, politicians, and civilians.
Dallaire's planned raids on suspected weapons caches and other preventative measures were rejected by the UN and his force was depleted rather than reinforced with enough ammunition to "last three minutes".
"We were three weeks into the genocide, with over 100,000 bodies already, and New York was questioning whether I had the right to protect civilians," he says.
Toward the end of his time in the country, the force commander said he had been actively seeking out ambushes to kill himself, and years later went public with a PTSD diagnosis.
"I felt that I had failed. The guilt took years of therapy. It's not enough to say you did your best when you command," he adds.
"How can you come to grips with the fact that you killed child soldiers and then face your own kids?"
Stressing the importance of peer support, Dallaire now puts his time into the Roméo Dallaire Child Soldiers Initiative, which helps to prevent the use of children as combatants.
UN Peacekeeping response
A UN spokesperson said: "UN Peacekeeping missions deploy in some of the world's most complex environments. Our peacekeepers serve with courage and determination, often at great risk, protecting millions and performing life-saving work.
"The UN continues to learn from its failures in Rwanda and Srebrenica, initiating key reforms since 2000 to improve its peacekeeping delivery and protection mandates.
"As part of recent reforms, the Secretary-General's Action for Peacekeeping initiative in partnership with member states is helping to improve performance and meet today's challenges.
"We continue working with our troop and police contributors to ensure our peacekeepers have the appropriate tools, technology and training to implement their mandates as well as the critical support required during deployment, including rapid and effective medical care for its personnel.
"Recognizing the mental health burden on our troops, we are currently in the process of developing, together with member states, a comprehensive support structure that entails prevention, detection, treatment and post-deployment care, which will be a key component of the ongoing Post Traumatic Stress Disorder project, along with an overall mental health strategy for uniformed personnel in the UN.
"We have a duty of care towards our personnel, and we take this responsibility to support them very seriously."