Although the broader stock market showed a solid effort to recoup losses from four consecutive sessions spent in the red heading into Monday, the trucking sector couldn't keep pace.

A number of the industry's biggest names traded sharply lower Monday as Wall Street grew increasingly fearful of a trade war that could follow President Donald Trump's plans to impose stricter tariffs on both steel and aluminum. Many investors have shown concern that the tariffs could hamper growth and raise consumer prices in the U.S. while dampening trade relationships the country has with foreign entities.

President Trump exacerbated concerns Sunday, March 4, when he tweeted that "friends and enemies have taken advantage of the U.S. for many years."

Investors are presumably concerned that the trucking industry could be among the hardest hit should trade policy enter even murkier waters. The sector will have a smaller volume of shipments should prices get higher for consumers or imports slow for businesses.

USA Truck Inc. (USAK) - Get Report shares fell 6.6%, Covenant Transportation Group Inc. (CVTI) - Get Report  slid 4%, Echo Global Logistics Inc. (ECHO) - Get Report shares lost 2.3%, J.B. Hunt Transport Services Inc. (JBHT) - Get Report stock dipped 2.4% and Werner Enterprises Inc. (WERN) - Get Report shares fell 2%.

While a trade war is in no way guaranteed, losses in the transportation industry could be a harbinger of market troubles to come.

According to the Dow theory, transportation stocks are considered the backbone of the economy. When they perform well, the whole economy is presumably doing well, too. Additionally, the theory holds that major trends should be confirmed by the Dow transports and Dow industrials indexes. If both maintain positive momentum, Wall Street takes it as a buy signal. But if both move lower, investors recognize a sell signal.