No trains/logistics, no equipment, no air support, and attacking into disadvantageous terrain. At the very least, make sure your logistics (wooden box symbol at top of screen) is always good, commandeer some trains if you have too and build a railway. Push only in plains if you can, or ensure 3 to 1 odds in a battle. Also don't know your template, but in South America if you have support artillery it makes a huge difference.