"Domestically, Britain was very divided on the American Civil War. There were economic issues, ideological issues, and moral issues involved and different segments of the British population were on opposing sides. So any intervention in the American Civil War would have caused a huge political crisis in Britain.
Strategically, Britain knew that the United States and the Monroe Doctrine worked to British advantage. Occupying the United States would have been a huge expense and would have forced Britain to either allow other Europeans to occupy their own shares of the Americas or to assume the burden of keeping them out. Either option would have stretched British resources and made Britain the target of more foreign resentment."
I just read this in a thread about why no one attacked the US during the Civil War (think about it, why wouldn't they attack a small country with rich economic resources in the middle of a crisis?).
My thought, on reading this, was "and that's why the US will do nothing when China, in the next few months, invades Taiwan."
We're too heavily involved elsewhere, and China owns too much US debt.