They seemed not to have a care in the world as 2018 began. Stocks recorded a string of record highs as Wall Street contemplated the benefits from reduced corporate and personal income tax rates that had just been enacted. But Treasury bond yields rose steadily through January as indicators of economic strength, particularly the persistently low unemployment rate, raised the prospect of rising inflation.

Around the same time, President Trump began to impose tariffs intended to protect American industries from foreign competition. Solar panels and washing machines were his first targets. Then came steel and aluminum.

Tariffs may or may not help the targeted industries, which in these cases are not significant segments of the economy, anyway, and the taxes are almost universally viewed as harmful to consumers, who have to pay more for the affected goods. And then there’s the impact of tariffs imposed by foreign governments in retaliation, which came swiftly from China, the main target of the trade war that Mr. Trump has threatened to wage.

The potential impact of the back-and-forth trade moves, and the ones that yet may come, helped send stocks on one of the swiftest plunges on record starting in late January — 11.8 percent in less than 10 trading days for the Standard & Poor’s 500-stock index.

That was followed by a rally that recouped most of the loss, and then another decline that took the index close to a fresh low, leaving it down 1.2 percent in the first three months of the year, the first quarterly decline since 2015. The early days of April have been rocky as well.