Prof. Barbara F. Walter explains in her book, “How Civil Wars Start,” that a civil war is very close when really important groups begin to fear losing their power and influence.
A political party, a minority group, a religious group, a current government, a key institution or organization, or even an antigovernment militia group, can be one of those groups.
Right now the leading group in the U.S. is the Republican Party.
Examples elsewhere around the world include Hindi and Muslim groups in India; Shia and Sunni groups in Iraq; Catholics and Protestants in Ireland; Walter lists and explains many more.
In today’s Internet and social media-driven world, Walter explains that civil wars usually begin with groups or inspired individuals making personal attacks… but they often also go on to include street protests, militia wargames or dry-run attacks, labor strikes, targeted assassinations and bombings, and more.
There are many ways to stop modern civil wars… these include finding leaders to call-out and explain over-and-over what’s happening; asking prominent reporters and respected pundits to urgently repeat every day the “democracy in danger” message; offering civics and media literacy education in schools and community groups. Walter’s book lists more.
Walter’s book is an argument that we in the U.S. are very close to a civil war… and it issues a strong warning that this is how other democracies have been lost.
I highly recommend this book… it is very well-researched, and her examples and practical suggestions are all based on historical facts.
Larry D. Lauer 
Another similar book is Marche’sThe Next Civil War which covers similar issues. It reviews how our hyper politics is creating a dangerous environment that can easily lead to violence which our military would have problems to control.