No way to expand

[Forward Observer]

Remember that whether a clan is Shogunate or Empire, all are nothing more than petty warlords who are basically looking out for #1. If it either bothers you to turn on an ally, or if it is the fact that said ally may presently be too strong to attack and/or also allied with others even stronger, then simply incite a revolt in their province closest to you. Chances are the rebels will take it, and then you can move in without declaring war. You can eliminate whole clans this way without ever being at war with them.

The downside is that sometimes the rebel army that spawns will be large, they will hole up in the castle, and after defeating them you will find everything quite damaged--which will present you with a substantial repair bill. However, it's still usually cheaper than attempting to build up an extra stack of troops and ships to chance a invasion of a distant enemy province--especially early in the game when your economy is just getting off the ground.

Cheers

P.S. If any of the damaged city structures don't fit your needs, don't hesitate to torch them instead of repairing. Then you can build to suit your particular gameplan on the next turn.

[Trithemius]

Sadly, if you want to bash your friendly neighbours you are pretty much required to be a sneaky jerk! ;)

In my game the Tanegashima got on a boat and went off to capture Kazusa - which made them the most effective of all of my allies really; you could always take a leaf from their book and "emigrate"?

[easytarget]

I've found what I've been doing in FOTS a lot more of is getting on a boat. Since the AI is more navy focused, I have been as well. As a result, I've got a pretty robust navy cruising around, and at the point where I inevitably get bottled in fairly early on by allies, I just continue building my economy, steadily crank out a couple stacks, put'em on a boat, and conduct a rear guard invasion of the opposing side. And I make a point of invading whoever is the strongest, or I suspect going to become the strongest clan among the opposition. Works like a charm.

[Voigtkampf]

Oddly enough, I dropped my focus from the navy entirely. Playing as Choshu, I focused on fortifying my assets, expand only slowly, one province at the time, and build up just enough forces to hold the province from invaders. I usually beat them to a bloody pulp when they assault my castle. Navy upkeep costs a lot of money, so I focused on ishin shishi and inciting rebellions in provinces behind enemy lines. Once my immediate neighbors find themselves with only a couple of provinces under their command, they lose the will to sail around my borders pretty fast.
When playing on harder difficulties, I found that the navy upkeep is better spent on the improvement of my infrastructure, and whereas I did in fact got bombarded few times, it was well worth the cost. Not to mention that once you've expanded you can go and build massive fleets within a blink of an eye and stomp everyone into the ground...Erm, water... Hmpf...

[Trithemius]

Oddly enough, I dropped my focus from the navy entirely. Playing as Choshu, I focused on fortifying my assets, expand only slowly, one province at the time, and build up just enough forces to hold the province from invaders. I usually beat them to a bloody pulp when they assault my castle. Navy upkeep costs a lot of money, so I focused on ishin shishi and inciting rebellions in provinces behind enemy lines. Once my immediate neighbors find themselves with only a couple of provinces under their command, they lose the will to sail around my borders pretty fast.
When playing on harder difficulties, I found that the navy upkeep is better spent on the improvement of my infrastructure, and whereas I did in fact got bombarded few times, it was well worth the cost. Not to mention that once you've expanded you can go and build massive fleets within a blink of an eye and stomp everyone into the ground...Erm, water... Hmpf...

I neglected navy for a while (was going for British trading port so didn't make a lot of other naval centres) and I suffered a lot! I ended up with a lot of exposed coast and repair bills from enemy bombardment. It got particularly hairy at one point with many of my farms and towns being set afire by the angry guns of my (many) enemies - trust me to ignore navy while my enemies focus on it! They even pulled the rear-guard naval invasion trick once or twice themselves, the dastards!

[Voigtkampf]

It got particularly hairy at one point with many of my farms and towns being set afire by the angry guns of my (many) enemies - trust me to ignore navy while my enemies focus on it!

I think you did just right, and old Sun Tzu would have agreed. You don't attack your enemy where he is the strongest, but where he is the weakest. Take land troops, destroy their provinces and infrastructure, incite rebellions, bribe, sabotage, kill and maim. Lets see how many ships they can keep running from the income they pull out of smoldering ruins, harr harr!

They even pulled the rear-guard naval invasion trick once or twice themselves, the dastards!

And I like this, the AI is much better in this segment than all the other TW titles before.

[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.