I agree with some of what you say, specifically the letters and diaries of the soldiers.
But I feel that most Northern soldiers were simply repeating a political platitude.
When the War began, half of all Americans had lived their entire lives within 100 miles of their birth place.
Hard to believe that very many of them had a realistic understanding of a “Union.”
Perhaps they were motivated by some sense of an “American Empire” expanding to the Pacific, but an inviolate Union with the Southern States, that’s very hard for me to believe.
Bottom line for me - slavery was the central issue of the War.
Take out slavery, there is no secession.
Take out slavery, there is no galvanizing reason to invade the South.
“But I feel that most Northern soldiers were simply repeating a political platitude.
You may be right. It’s a complex question with a million motivations and we’re trying to distil it down to 1. In the end, I believe that most Union soldiers were conscripts, so it may be safe to say that most went because they were forced.
“Perhaps they were motivated by some sense of an “American Empire”...”
Great observation. Remember our Manifest Destiny and the beautiful Columbia.
“Bottom line for me - slavery was the central issue of the War.”
It always comes down to that. And people knew it at the time. Even before the Emancipation Proclamation. I think people today forget or ignore the 100+ years of abolition. It was a powerful political movement. President Lincoln didn’t just make it up.