Playing as Tosa with a bunch of allies (not just trading partners) around me, I simply hopped on fleets on the north and south coasts and took my fight to those Shogun-boot-lickers in person. It was great. I obtained military access from a little clan in the north and marched an army down from the province just below that hook, and then plopped a powerful army right on Edo's doorstep. No problem! But I was also playing a short campaign.