Marines all have an M16A4 with 5 total mags each. They also have a War Ace. Argonians have leather/iron armor, depending on rank. There's 2,500 iron-clad officers, and 7,500 pigskins. U.S. Marines have no body armor. Battle takes place in Black Marsh. --- http://i.imgur.com/ckdCO.jpg http://hdscreenshots.avatarspiritmedia.net/korra/109/1861.jpg
Humans would die within a week due to foreign diseases they never built up a resistance to. Argonians drink swamp juice and watch the soldiers fever up and die, they never even have to lift a finger.
Assuming that they wanted to, though, they'd just ambush them, or zerg them. Either way, the soldiers are screwed. --- Ajd of House Ajdatheon, the First of his Name, King of the Posters and the Lurkers and the Troll Men, Lord of the Seven Platforms and Protector of the Forum.
15,000? That's a bit too excessive. The Marines would not even have enough ammunition to kill 15,000 Argonians assuming the kill ratio is 1 bullet: 1 kill. Many would probably die from diseases and other environmental hazards in Black Marsh. If you factor magic in as well...