wish the wiki was better, now started as America and went fascist. conquered most of south America, but got terrible attrition and not sure how to get less attrition.
South America is basically mountains, jungle and/or really hot which all cause attrition. There is also quite poor infrastructure which hinders the supply lines, causing more attrition. You can always click the supply overlay (F4 i think) and it will tell you how many brigades a region can support without supply attrition. If this is the case all you can do is move brigades out of the region, repair the infrastructure (it's usually damaged by combat), upgrade it via construction menu or get politicians/support companies to reduce it . If you create a battle plan and some units are being held back with a yellow exclamation point it means that there is not enough supply in the region they are supposed to go to and the AI is holding them back.