(CNBC) — The Dow Jones industrial average fell on Monday as investors worried a trade war could develop after President Donald Trump implemented tariffs on steel and aluminum imports.

The 30-stock index declined 156 points, with Boeing, Caterpillar and United Technologies all falling at least 2 percent. The Dow rose more than 100 points earlier in the session as shares of Apple and Goldman Sachs reached all-time highs.

“It’s a growing concern that a mistake in trade policy could be as big as a mistake in monetary policy,” said Art Hogan, chief market strategist at B. Riley FBR.

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