Court Martial Centre in Bulford (Picture: PA).
Two Special Forces instructors have gone on trial by court martial over the deaths of three reservists on a 16-mile march in the Brecon Beacons.
The defendants, know as '1A' and '1B', both deny "negligently performing a duty" by failing to take reasonable care for the health and safety of candidates taking part in the exercises.
Lance Corporal Craig Roberts and Lance Corporal Edward Maher were pronounced dead on the Welsh mountain range after suffering heatstroke on 13 July 2013.
Corporal James Dunsby died at Birmingham's Queen Elizabeth Hospital from multiple organ failure more than two weeks later.
1A and 1B, whose identities are protected by an anonymity order, went on trial at the Court Martial Centre in Bulford, Wiltshire, on Monday.
1A was a Captain and the training officer in charge of the march, while 1B was a Warrant Officer and the chief instructor on the exercise.
Prosecuting, Louis Mably QC told the five-person board: "In those respective roles, they were the safety officers for the exercise.
"The charge against the defendants is that they were negligent in their duty to plan and conduct the exercise safely in relation to the well-known risk of heat illness.
"The risk to the candidates wasn't properly appreciated or assessed and in this regard there was a negligent failure with planning and preparation.
"Two reservist candidates collapsed on the road from heat illness and died where they fell. A third reservist who also collapsed on the road later died in hospital.
"There were also a number of non-fatal heat illness casualties."
Mr Mably told the board that the risk of heat illness was well known and "the defendants ought to have taken proper attention" to it. He said:
"It is of course the case that with an arduous endurance exercise the risk of heat illness cannot be eliminated completely.
"But the risk did need to be properly evaluated by the defendants. They needed to prepare and plan for it and on the ground, they needed to be ready to act in a dynamic way if the risk arose.
"When things began to unfold on the Brecon Beacons, they were in no position to respond with any real effect. What the prosecution say is that this was negligent."
The court heard 1B filled out the risk assessment form for the exercise and it was signed by 1A in June 2013, the month before it took place.
Following the deaths, a number of inquiries took place and systemic failings were identified, the court heard.
"The defendants' personal failings were basic and fundamental," Mr Mably said.
"They did not do what they reasonably could and should have done.
"They did not carry out their duties with the diligence and care that they should have done."
37 reservists and 41 regular troops took part in the exercise, which was part of the aptitude phase for selection for a special military unit.
The march was 16 miles as the crow flies - though those taking part were expected to cover almost 18.5 miles - and had to be completed in eight hours 45 minutes.
Candidates carried a bergen, a backpack, weighing between 22 and 27kg as well as a dummy rifle.
Those who voluntarily withdrew from the march, or were withdrawn on medical grounds, failed - providing an incentive to "keep on going", Mr Mably said.
Temperatures reached 26.3C from midday on the day of the march and had risen to 28.3C by 4pm.
The Wet Bulb temperature, which takes factors such as wind and humidity into account, was 31.2C, the court heard. Mr Mably said:
"Had the defendants obtained that kind of reading and been aware of the conditions under, they would have been alert to the very real risk that the exercise was about to be affected by heat illness.
"That kind of reading indicates that they were operating at or beyond the limits. There were very significant failings in this regard."
The court martial continues.