Army of defeated enemy still exists after all provinces have been taken

Hello!

I defeated and took every province from an enemy. South Argentina. Game #2519577 I am South Africa in the game. After I moved on to Bolivia. I am surprised to find the 43 stack army of South Argentina in Bolivias capital and its artilleries were firing at me. I'm just shocked because normally after you take every province the player is defeated and the armies are supposed to disappear. Please help. Thanks