1,000 soldiers take part in Welsh mountains challenge
About 1,000 soldiers from 34 countries are competing in a major mountain exercise to test their fitness and military skills.
The 37-mile (60km) Cambrian Patrol also sees soldiers face scenarios including "enemy" forces, in the Brecon Beacons.
They will need map skills and endurance to navigate Welsh mountains over 48 hours with a 50lb (22.7kg) pack.
This year marks the 60th anniversary of the exercise organised by the 160th Welsh Brigade, based in Brecon.
Brig Alan Richmond, head of the Army in Wales, said the exercise was a "very good gauge to test the basic soldiering skills".
"What is clear today is the esteem to which international armies from across the world hold this event," he said.
"Many see it as the ultimate patrolling challenge and this year we have 34 countries visiting us to take on the challenge - that's a record and shows how far this event has come on from the days when it was classed as a march."
British soldiers and reservists will be up against counterparts from France, Switzerland, Lithuania, Nepal and Greece among others over the next week.
A patrol from the Australian Army won an infantry skills competition to represent their country on the march.
It started in 1959 with a march organised by the Welsh Territorial Army (TA) along a 74-mile (119km) route over many of the peaks of the Black Mountains and Brecon Beacons from Hay-on-Wye to Maesteg.
This year's exercise runs from Friday with patrols starting off on different days and times, all culminating in daily completion ceremonies at Sennybridge Training Area camp from Sunday and until 20 October.
These include casualty evacuation scenarios and dealing with issues such as finding mock explosive devices.
Patrols have to finish within 48 hours with at least five members.
Former TA officer, Lt Glyn Powell, 87, who took part in one of the patrols in 1961, warned this year's entrants to look after their feet.
"Some of the boys used to cut holes into the front of their boots when the pain got too much," said the retired headmaster who still lives in Sennybridge.
"It was as much a question of mental strength as it was physical."