Originally Posted by alpaca
Going around in the north is not an option as you'd have to shuffle all your units through a single tile mountain pass.
In my opinion you have three options: 1) use infantry and a lot of artillery in a combined force manner, or 2) circle around over the ocean in the south (you can't see it but Monaco there on the southern tip of the continent is actually allied with Japan, which they bought just before they declared war), or 3) sign peace, open borders, and attack somebody else. What would you do if you weren't playing against an AI who'll just throw their units against you?
I think it depends on the two city states in the north - if they are not allied with Korea, Oda could get them through bribing and/or espionage, and then use them as a base for his attacks, coming rather from north-east then directly from the north. This would need some manoeuvering, but would still be faster than going by the sea route in the south you mention.