After watching HBO's "The Pacific" I began reading Eugene Sledge's With the Old Breed, one of the books on which the series was based. A passage I read with interest is where Sledge observes a fresh batch of Army boys coming in to relieve his battered Marine platoon and he comments that two of the new guys are wearing eyeglasses, which he hadn't seen a soldier wearing before.
After reading the passage I removed my own eyeglasses and inspected them. I cannot imagine going through the hell of the Pacific war, or any military maneuver for that matter, with these delicate things perched on the front of my face.
Neither can the U.S. military, at least not in Basic Training; a little research led me to GI glasses, which are handed out to Armed Forces recruits at recruit training, where contact lenses and civilian eyeglasses are strictly prohibited.
Design-wise, the emphasis on GI glasses is for maximum durability at minimum cost. They're made of brown acetate plastic, are impact-resistant, waterproof, and ugly as sin. Reflecting this last point is their nickname--BCG's, or Birth-Control Glasses.
For those with a taste for them, it is possible to buy a pair without signing up for Uncle Sam; some civilian optometrists reportedly sell them in the bargain section, and eBay's got tons of them--though sadly overpriced at anywhere from $45 to nearly $100.