Buried in a recent spending bill approved by Congress was a poison pill — a provision terminating U.S. access for 29 Mexican trucking companies.

Following Congress’ misguided move, Mexico has decided to retaliate: It slapped tariffs on $2.4 billion in U.S. products from 40 states.

That’s a shame. The pilot program killed by Congress was an attempt to move toward implementation of the long-delayed cross-border trucking provisions of the North American Free Trade Agreement.

In 2001, a five-member arbitration panel unanimously ruled that Washington was in violation of NAFTA.

Under the pact, U.S. and Mexican trucks were to have reciprocal access to each other’s highway networks.

The 2007 pilot program significantly lowered transportation costs, since loads no longer had to be transferred to warehouses, then reloaded onto U.S. trucks.

The Department of Transportation calculated that the needless unloading and loading added $400 million to the price of Mexican imports — costs that were passed on to U.S. consumers.

Since the signing of NAFTA in 1994, safety concerns about cross-border trucking blocked efforts to fully implement its terms. But Mexican trucks participating in the program had to meet U.S. Department of Transportation safety requirements and were subject to monitoring while in the United States.

Last year, the Federal Motor Carrier Safety Administration said no accidents involving participating trucks had occurred. Objections to the program had less to do with safety than with union opposition to competition from Mexican drivers.

The Obama administration says it will press to restore the program, but quick action isn’t likely. Meanwhile, U.S. producers subject to the tariffs will stand to lose a portion of the Mexican market, hardly desirable in a deep recession.

The larger danger is that with both the United States and Mexico in recession, low-level disputes of this sort could lead to bigger problems.

Mexican congressional elections are scheduled for July, and heightened anti-Americanism could produce more lawmakers who favor protectionist policies.

That would be bad for both countries, and would undermine cooperation on a host of issues such as the drug trade. The Obama administration must work to make sure that these tensions lessen.