Speaking at a campaign rally in Ohio last weekend, President Donald Trump said foreign companies that sell goods in the United States would have roughly 18 months to build factories in the U.S. or potentially face tariffs ranging from 150% to 300%. He pointed to countries including South Korea, China, Japan, and Canada while arguing that tariffs are forcing foreign companies to invest in American manufacturing.

The threat comes less than two weeks after Trump met Chinese President Xi Jinping in Washington. The market had hoped for more progress toward ending the trade fight between the world's two largest economies. It didn't get much.

Image source: Getty Images.

The U.S. and China agreed to extend their existing trade truce for another two months, pushing it into January 2027. They also agreed to pursue lower tariffs on about $30 billion of goods in each direction and establish additional channels for discussing trade and artificial intelligence (AI).

But many of the biggest issues remain unresolved, including broader tariffs, technology restrictions, Chinese purchases of American agricultural products, and access to rare-earth materials. In other words, the summit bought more time. It didn't end the trade war.

Why Wall Street cares

The problem for the market isn't simply whether Trump ultimately imposes a 300% tariff. It's that companies don't know what the tariff landscape will look like in six months or a year.

A company deciding where to build a factory, source components, or sign a long-term supplier contract typically makes those decisions years in advance. A tariff that suddenly makes imported products 50%, 100%, or 300% more expensive can completely change the economics. And we've already seen how quickly markets can react.

When Trump threatened new tariffs against several European countries earlier this year, the S&P 500 (^GSPC -0.22%) fell 2.1% in a single session, while the Nasdaq Composite (^IXIC -0.22%) dropped 2.4%. Reuters attributed the broad sell-off largely to renewed concerns about trade uncertainty.

Tariffs can also squeeze corporate profit margins. Companies importing products or components generally have three choices: absorb the additional cost, raise prices, or move production. None of those things happens overnight.

And while Trump's strategy is clearly designed to encourage more manufacturing inside the United States, even companies willing to move production need factories, equipment, workers, suppliers, permits, and capital. That's why you can't dismiss a 300% tariff threat simply because it may never become a 300% tariff.

The uncertainty isn't going away

Markets have actually remained remarkably resilient despite so much tariff uncertainty. The S&P 500 entered October up nearly 13% for the year, while the Nasdaq recently hit another record high.

But the Trump-Xi summit didn't produce the broad trade agreement the market has been waiting for. And Trump's latest warning makes it clear that tariffs remain one of his preferred tools for pushing investment into the U.S. But as long as companies can't predict where those tariffs are headed next, trade policy will remain a frustrating and potentially dangerous wild card hanging over the market.