War choices

Hi guys. I have a dilemma and would like a little advice.
I am playing on a continents map with domination and cultural victories enabled. I am playing as Ethiopia against America, the Zulus plus a few others. I am currently sharing a continent with just America and we are good friends and trading well. The problem is that Washington is expanding incredibly well and starting to accelerate away in general score compared to me. He definitely needs to be stopped and I do have the edge on the military as we are roughly equal in quantities but I have researched dynamite and built a few artillery units. I was just setting said units in place for a surprise attack when Shaka started to turn ugly (there's a surprise). He is hostile, insulting me and demanding tribute from my city state allies. Also he has a large army. Should I.....
A) Attack America anyway and damage him enough to stop his growing threat.
B) Send my units back to defend my coastal cities as an attack from Shaka is likely.
C) Build a naval fleet and hunt Shaka down as he is really grating on me now. His score is also decent.
Thanks in advance for any tips and advice.

Shaka's on another continent and the AI generally isn't as good at naval warfare as they are at land warfare. Option A is probably best. Your coastal cities alone plus a few ships/ranged units should be able to defend you for a while if Shaka does attack.

The only issue is that you're possibly betraying a longtime friend of yours. I assume you've thought long and hard about it anyway.

If Shaka is on another continent then I would build a few naval units to stand sentry in the ocean and see if he embarks units. Build a big enough navy to sack a city.
Start your war with America. Finish as quickly as humanly possible.
If Shaka starts embarking units. Encircle him with your navy and attack the units while their still in the water preemptively. Immediately beeline to his biggest coastal city afterwards. Position yourself as if you will attack. If you have a big enough show of force he may be willing to take peace. If not take the city. If he has a lot of coastal cities. Don't even worry about guarding the city you take. Just sack each city on his coast. If 3 or 4 cities fall, you will cripple his economy and possibly production. Even if he wins his cities back, his cities pop is now 25% of what it was with practically no buildings left. Also I sometimes like to be a douche and sell or gift the cities to neutral factions who end liking you alittle more and possibly razing the city for you.

what skill level is this? What about bribing America to was Shaka or other way around? Anything to distract America? Or attack Shaka's coastal cities gift to America so they now have a direct border so America will be more likely to actively fight him

Thanks for the replies. I did attack America and virtually wiped Washington out. The threat from Shaka didn't materialise and I went on to win a cultural victory without having to face him at all!