I've been listening to a podcast the dwells deep into the background and causes for WWII. They spent some time going over the hyperinflation that Germany went through from 1921-23. While we are not saddled with the crippling war reparations Germany had, some of the other issues such as ever increasing debt servicing costs give me concern.
Well, Germany was forced to pay war reparations, in gold, then in production. That took money out of the active economy that would normally go to labor and investment. If your business has half of its gross taken away, you would be hard pressed to stay in business. In short, their debt to the victors was very real.
As I understand it, Germany, chafing under reparations, started to print money and pay those reparations in increasingly depreciated money. If you print up a bunch of money and don't spend it into the economy, where it should lead to increased production, it's going to lose value.
Germany recovered from the Depression faster than other European countries by deficit spending on arms production. They built up their domestic economy, and were no longer paying reparations. Sovereign "debt" is just another way for the government to create and spend money. It's a mistake to just give it away when there is no extra production to buy, but it's smart to invest in your economy like they did.
During WWII, the U.S. and U.K. governments both basically took over industry to build tanks and planes. Private sector investment, already low from the Depression, went down further, but was replaced with government deficit spending. The government paid for the labor and the materials, and stuff got made. There wasn't much for people to spend money on anyway. After the war, the government scaled back on spending and private sector investment took off, and things were back to normal. And all of that deficit spending didn't get "paid back," the bonds remained as savings, or people cashed them in and bought cars and houses.
Debt service looks bad, but it's just money going to bondholders. It doesn't disappear unless you run a tax surplus, and there's no good reason to extinguish those bonds anyway.