The US Stock Market’s Tariff Exposure Is About to Be Laid Bare

President Donald Trump’s raft of tariffs is set to expose just how vulnerable the US stock market is to a trade war.

The track record of S&P 500 Index companies over the past two decades suggests their ability to withstand additional levies is fragile, at least by one measure. Nearly all of the margin growth eked out from corporate sales on the gauge since 2004 has come from the booming technology sector, according to Bloomberg Intelligence. Removing the group, profitability barely rose.

The consequences for the US economy and corporate profits from the proposed tariffs is one of ...