I think the problem is Korea's population is way too high in vanilla Vic, this make Korea a lot harder to beat than they should be, but it also makes the rewards much greater. Korea's population is fixed in VIP, and in 0.3 alpha AI Japan can stomp all over AI Korea.
lzprst: Your army was rather small and it sounds like Korea mobilixed while you didn't, a reserve pool would help you even the odds. Your troops should also be more power than Korea's due to your higher tech, although a just civilized Japan might not out tech Korea by that much. It looks like you should have either have waited longer to build a larger army and increase tech (you could speed this up by trading for some army techs, but rember they'll still cost RP) or taken your small army and dug in in a hilly/mountain area and let Korea bleed itself on your entrenchments.
lzprst: Your army was rather small and it sounds like Korea mobilixed while you didn't, a reserve pool would help you even the odds. Your troops should also be more power than Korea's due to your higher tech, although a just civilized Japan might not out tech Korea by that much. It looks like you should have either have waited longer to build a larger army and increase tech (you could speed this up by trading for some army techs, but rember they'll still cost RP) or taken your small army and dug in in a hilly/mountain area and let Korea bleed itself on your entrenchments.