The New American Civil War, also sometimes called the Second American Civil War , has not broken out, as of June 2018, although some (such as reporter Carl Bernstein ) have claimed, or observed, is that the American populace is suffering through a Cold Civil War  or Soft Civil War.  The present article is not about any fictional "hot" civil war--which is by its nature speculative and unknowable--but about the current discussion and speculation about such a conflict.
Why think a New Civil War might break out?
Discussion of a New American Civil War is, as of 2018, a staple of opinion pages. In a typical piece, a columnist explains:
[S]ome are saying we're heading toward actual civil war. There's evidence they may be right.
States are banning travel to other states, and some are threatening secession. Intolerant business owners are forbidding service to customers, based on political or religious views. Congressmen have been shot. 
Journalist Thomas E. Ricks reported that he "asked a group of smart national security thinkers" over a meal, and found that the answers ranged from five percent to 95%, with a consensus of "about 35%."  Another commentator put the probability at 60%, citing, among other things,
- "Entrenched national polarization of our citizenry with no obvious meeting place."
- "Press and information flow is more and more deliberately divisive, and it's increasingly easy to put out bad info and incitement."
- "Violence is 'in' as a method to solve disputes and get one's way." 
Hollywood has basically turned its products, and its award shows, into showcases for "the resistance." Americans are already sorting themselves into communities that are predominantly red or blue. And in heavily blue Washington, D.C. , Trump staffers find that a lot of people don't want to date them because of their politics.
What might spark violence?
Editorial writer Francis Wilkinson Pickens speculates that, if Democrats lose the 2018 midterm elections, "Democrats will then experience rage--at Tea Party levels or worse," and then "Democrats will give up on conservatives. ... They will explore ways to divorce their culture, politics and economy from Trumpism and from their fellow Americans who support it." 
A former special forces officer turned diplomat, Keith Mines, listed some other "events that could spark it," including these:
- Impeachment of the President .
- Major terrorist attack .
- "Opaque call to action by a failing president." 
Timeline of relevant events
This list needs to be greatly expanded with earlier events, e.g., the shooting of Steve Scalise , the election of Donald Trump, and notable instances of growing political intolerance (e.g., interruptions of speeches, increasing nastiness on social media, etc.).
June 23, 2018 - Sanders tweeted that she was refused service by the Red Hen because she works for Trump. 
June 24, 2018 - U.S. Rep. Maxine Waters (D-Calif.) called on citizens to harass Trump administration officials whenever spotted in public: "If you see anybody from that Cabinet in a restaurant, in a department store, at a gasoline station, you get out and you create a crowd and you push back on them, and you tell them they're not welcome anymore, anywhere." A video of Waters' speech was posted and commented on very widely. 
June 25, 2018 - President Donald Trump responded to Waters' speech with a tweet that read, in part, "She has just called for harm to supporters, of which there are many, of the Make America Great Again movement. Be careful what you wish for Max!"