Play as Estonia!
But seriously, you can make good territorial gains as a minor if you play right.
Don't attack right off the bat, build up your forces first- I suspect that may be why you lost against Argentina.
Also, be wary of Spheres of Influence. It's not too hard to get a list of nations that aren't in anyone's sphere. Haiti, China, Turkey, Saudi Arabia, Sinkaing, Spain, and several others are IIRC free to attack. If you desire to attack a nation that is in a sphere, however, save and reload right before you declare war, so the SoI events are started over (they only check once a few days.) If you get in a war with Italy, UK, Germany, or the USSR that way, its best to start over.
If you wish to invade a country that only has one Nation's Sphere on it, you probably should ally with the nation first.
If the USA declares war on you via SoI, if their WE level is low, you're usually safe. You can make peace with them after you annex whatever nation you attacked. Just don't send any ships out wandering too far, the US has a killer navy. And if the US lands an invasion force, its probably best to Reload.