President Trump’s retreat on tariffs calls to mind Churchill’s jibe in 1956 after Britain’s Suez debacle: “I would never have dared; and if I had dared, I would certainly never have dared stop.” That foreign policy fiasco signaled the end of British pretensions to be a global power. While Mr. Trump’s reversal on tariffs isn’t a calamity on a par with Suez, the episode does raise concerns that the national debt served to hamstring the president’s plans on a key policy.