The U.S wasn’t really implementing an isolationist policy, and never has since its inception. (Certainly not prior to WW1 when they had just finished going to war with Spain to take over its colonies, nor during nor after WW1 when they sent troops to a different Latin American country every single year to impose their will). It was just brutal realpolitik.

“Isolationism” vs free trade and free markets, the US government and ruling class just does whatever benefits itself the most. Hence, other countries need to open up their markets to US exports post WW1 but the U.S. will simultaneously levy protectionist tariffs so that European goods can’t be competitive in the US market. Germany had no recourse but to borrow more money from US banks to pay their reparations, so that UK can turn around and give that money back to the US government. The only other way for these governments to meet their payments to the US was to impose austerity and wring the money out of their own domestic population (which they also did, also a contributing factor to the turmoil which eventually led to another world war).

I don’t buy this “aw shucks we would love to forgive the debt or interest but we just can’t sell that to our domestic masses who care a lot about fiscal policy”. They did it because it directly benefited them (the ruling class and their state). They made massive profits off of the entire arrangement. Nothing mysterious about their motivations there. A better topic of discussion is would the European powers agreed to pay up, when that actually went against their own interests (look where it got them!)