In just two short paragraphs, the White House announced its intention to withdraw from a treaty it first signed during the Ulysses S. Grant administration: the Universal Postal Union. In doing so, President Trump opened a new front in his bid to rebalance trade between the United States and China.

At issue are the rates that foreign postal services pay the U.S. Postal Service to allow shippers abroad to send mail and small parcels to American customers. Since international mail necessarily involves two post offices, both of them need to be paid to have a reason to receive and deliver mail across borders. The UPU’s terminal dues framework coordinates negotiations between the world’s 192 postal operators, avoiding the need for individual treaties between every pair of countries that wish to allow mail to be exchanged between them.