There is no need for war. We can divide this worn out union up along state boundaries and let the internal affairs fix themselves. I expect much migration between the two new countries.
We are more like Spain in 1936 than the United States in 1861. There are conservative enclaves in blue states, e.g., eastern Washington and upstate New York. Conversely, there are liberal cities or enclaves in conservative states: Austin in Texas or Jackson Hole in Wyoming. If, say, Oklahoma were to secede, there would be leftist enclaves that could engage in sabotage. Even in super blue cities like Boston or San Francisco, there are patriots who would fight back. While the secession of 1861 was sometimes called a brothers’ war, that was more of a factor in the Border States. In the core areas of both the Union and the Confederacy, there were few volunteers for the opposite side.