America’s fiscal position is increasingly looking like a train hurtling along a twisting mountain track. No one seems willing to slow down for fear of falling behind schedule and losing their job. Even if the train doesn’t crash, it is worth asking: Why keep pushing it? Despite solid growth, the federal deficit is about 6% of GDP. Yet the option of slowing down does not appear to be in President Donald Trump’s playbook.

The start of a public-debt crisis is usually pretty straightforward. The end is much harder to predict. A crisis can erupt when a country’s debt is historically high but has re...