When Aetna, the health insurance giant, announced this month that it was pulling out of the Affordable Care Act's insurance exchange in Virginia in 2018, President Trump responded on Twitter: "Death spiral!" When Humana announced plans to leave all the health law's marketplaces next year, the president chimed in, "Obamacare continues to fail." Left unremarked on was a big reason for the instability: The Trump administration and Congress are rattling the markets. The administration's refusal to guarantee payment of subsidies to health insurance companies, the murky outlook for the Affordable Care Act in Congress and doubts about enforcement of the...