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[phred]

Playing as Choshu I built up my navy a little just to keep the coastline clear. I attacked a Nagaoka fleet that was blockading a trade partner and suddenly I was at war with Obama, Sendai and Nagaoka. oops.
They sent a steady stream of ships to harass me. Fortunately they sent ships one at a time and I was able to send out fleets of 3 or so ships to defeat them. Eventually I got wise and put a general on board. He leveled up very, very quickly.

To the OP: you should totally consider attacking fellow Imperials. In my current game Satsuma rampaged through Kyushu attacking everyone. Now they've declared war on Tosa and I imagine the Choshu (me) are next. I should have crushed them when they were smaller, but I've been a good ally. You should consider uniting Kyushu. fertile land, coal, copper are hard to resist.