when he leads his black soldiers to death, (39)
The company persevered all the way until the bitter end. Let's face it, this is not a great reward for their perseverance, but it's important to keep in mind that their side (the Union) was victorious, so they didn't die for nothing.
the old white churches hold their airof sparse, sincere rebellion (42–43)
The structures in these old New England towns are trying to hold on to their rebellious identity. They rebelled against England and the church of England, and they are trying to hang on to those roots.
Colonel Shawis riding on his bubble, (61–62)
Even in this imagined aftermath, Shaw just keeps on tickin'. We can't help but imagine this guy for a second when Lowell mentions the bubble, but we're guessing he means it more figuratively.