A simple question, but an answer that's perhaps not so simple.
In the 1800's there was an issue that divided the public, that of slavery. The US avoided a war because the two main parties were both divided over the issue. That changed in the late 1850's, and early 1860's when the parties themselves changed. Those Democrats who opposed slavery abandoned the party to support the new Republican Party, while members of the Whigs also joined the Republicans, with pro-slavery Whigs ending up in the Democrats. This gave the US a 2 party system, where each party had a clear majority of members who were willing to fight and die over this single, important issue. The result was civil war.
Today, many look at politics, and see that we have no over-riding issue, and two mainstream parties that have good relations with one another, and no desire to escalate things.
The reality is that for the past 20 years, the Republican Party has been moving, slowly but steadily, to the right. Recently, the Democratic Party has responded, and this creates a problem. Just as voters influence their parties, parties influence their voters. More and more, people in the US are moving away from the centre, and are moving towards dual-peak politics. This is not about disagreements between the party brass of one side with the brass of the other; it's a grassroots hate.
Here's the problem. Things do not appear to be changing, nor, is there any reason to think they will change any time soon. The two parties will continue to move further and further away from one another. This is not simply a problem is ideology, it's a problem of thought.
Consider; for example, climate change. Lets create a fantasy world to help us examine this. In this world there are two countries, one is full of people who believe in climate change, and the other is not. The problem is that first country is near sea level, and will be drowned by rising seas. The second country, thinking that all of this is nonsense, continues to dump pollution into the air. This, is a recipe for war. It becomes, truly, life or death, and about survival.
Fortunately, at this time, there is no one single issue that both sides are willing to die over, but when you have a split so great that people live in different realities (including one where thousands of muslims were celebrating 9-11 in the USA) it does not take long before some future issue can turn into something worth fighting over.
There are still a number of ways to avoid a civil war.
1 - The easiest way to solve this is to create a new political party. The fact that the 2 parties in a 2 party system are so far from one another is what fuels this. Creating a third party, one that find itself in a better consensus with the public is perhaps the best way to avoid these problems. There are ways to get here, and issues that may help. The idea behind a basic income is something that, if proposed by a libertarian, could end up with enough support on both the right and the left to support it's own party.
2 - A more difficult, but perhaps more politically plausible way to deal with this is to have the two parties stop their movement away from one another. This will depend on changing the rhetoric. We need to stop viewing those from the other party as "evil". This started with Bill Clinton and his infidelity. In response, George W Bush was made out of be the devil himself by many on the left. Obama now is seen as some kind of communist muslim by many on the right. If we are to heal this rift, we need to stop this nonsense.
3 - Time. It will be at least 15 years, likely 20, before this even becomes possible. That is much time to find another way to resolve this. We won't be seeing any US civil war prior to 2030. Simply sit and think what you were doing in 2000 to realize how long of a time that actually is. Time can solve this problem, and hopefully, will.