Even if it would be peaceful domestically

Just think about how much new international links and treaties would have come to be. Because like it or not, full isolationism wouldn't probably work in this day and age - at least economically. Trade must go on at least to some minimal extent.

Therefore, the new nation would have to have large enough economical AND military prowess for the world nations to come into agreements and treaties. Why?

Because old USA will surely extort the hell out of other nations to not do this, in the end isolating new nation from the world and Navy would surely try blocking all maritime routes they can, just like they did during the War.

The new nation/country needs to be big enough "challenger" to old US for their former allies/potential allies to reconsider their friendships. Otherwise, the part that cut itself from the US would surely be bullied into submission by any "peaceful" (economic) means necessary, or otherwise be ripe for adversary (Chy-nuh among others) to invade.

But I might just be wrong faggot and talking BS