Let's say that in the U.S., the rural Midwest along with a chunk of the rural South and Alaska seceded form the Union. Let's say the rural Midwest plus the Dakotas all the way down to the Gulf of Mexico (including Texas) plus Alaska. They're not trying to overthrow the existing government of the U.S. and dictate life to the remaining part of the country, simply trying to leave and start their own government. Now, their reasons for doing so have nothing to do with anything anywhere near as morally reprehensible as slavery. Let's say it actually IS economic reasons that caused this situation in this particular case. If you were President of the U.S., would you send in the tanks, the infantry, and the planes to carpet bomb cities like Minneapolis MN and Lincoln NE to the ground and basically kill enough of them to the point where they agree to come back against their will in chains and vow to be part of your nation again as long as you promise to stop killing them and their families? I would suggest a non-violent alternative. We could use economic sanctions to prevent them from trading with anyone. That's right, no goods in or out of the their country. We would starve them out. They would come back before their families died and beg to become part of the Union again. They would have only an isolated geographical region. There are certain crops such as olives, coconuts, and pineapples that they simply cannot grow in sufficient numbers. I would use their lack of sufficient agricultural diversity to make them understand why we need each other.