When interest rates are near zero, government debt doesn’t matter. That was the theory at least, in the previous decade, not just from economists on the fringe but also from a few in the mainstream. If rates stayed low, the argument went, the US didn’t need to worry about debt. And it didn’t. Debt increased, and economists who cautioned against it (ahem) were dismissed as cranks, with many of our critics pointing to the history of one nation: Japan, which managed to keep interest rates low even as its debt grew to more than 200% of its GDP. Reality, however, has a way of catching up with theory eventually, and now it has for Japan. If it increased rates to fight inflation, then debt costs would balloon. If it kept rates low and let inflation run high, the yen would depreciate. In the end, it chose the second option. The timing couldn’t be worse, with Japan mulling more fiscal expansion to revamp its economy, and it’s now risking a full-on debt crisis. The lesson here is that...