According to the PBS documentary, "Crucible of Empire", during the landing in Cuba at the start of the Spanish American war, the Rough Riders stripped naked and finished the landing wearing only their cowboy hats, after their uniforms had been soaked in the surf. When else have so many naked men been on the front line?
Here's a link to the documentary, they discuss the landing at around 1:00, with photos and witness accounts. https://www.youtube.com/watch?v=EiMRo8CBLCY
Where on earth did you hear such a thing?
The ancient Celts often (but not uniformly) fought in the nude.
At the Battle of Telamon, the Romans faced a confederation of Celtic tribes. One tribe, the Gaesatae, chose to fight naked and Polybius describes them:
Not less terrifying was the appearance and rapid movement of the naked warriors in the van, which indicated men in the prime of their strength and beauty: while all the warriors in the front ranks were richly adorned with gold necklaces and bracelets. These sights certainly dismayed the Romans; still the hope they gave of a profitable victory redoubled their eagerness for the battle.