The civil war was definitely about slavery.

main.rs

use regex::Regex;
use std::{env, fs};
use std::collections::HashMap;

fn main() {
    let args: Vec<String> = env::args().collect();
    let filename = &args[1];
    let contents = fs::read_to_string(filename)
        .expect("Something went wrong reading the file");
    let re = Regex::new(r"\b[a-zA-z]*").unwrap();
    let mut word_count = HashMap::<String, u32>::new();
    for word in contents.to_lowercase().split_whitespace() {
        for captures in re.captures_iter(word) {
            let word = &captures[0].to_owned();
            if word.len() < 5 {
                continue;
            } 

            if word_count.contains_key(word) {
                word_count.insert(word.to_string(), word_count[word] + 1);
            } else {
                word_count.insert(word.to_string(), 1);
            }
        }
    }

    let mut count_vec: Vec<(&String, &u32)> = word_count.iter().collect();
    count_vec.sort_by(|a, b| b.1.cmp(a.1));

    println!("{:?}", count_vec);
}

Written to end a facebook comment argument about southern secession. TL;DR "slave", and "slavery" are very common words in the articles of secession.