Secretary of State Marco Rubio embarks soon on his inaugural trip as the United States’ top diplomat. His first stop, Panama could prove to be the most contentious on the itinerary.

During Monday’s inauguration, Donald Trump repeated his threat to retake the Panama Canal. The United States controlled the waterway since the early 20th century, but in 1977 President Jimmy Carter signed a landmark treaty to give Panama control of the canal.

The neutrality of the nearly 50-mile canal, through which nearly 15,000 ships transit each year, is enshrined in Panama’s Constitution and is enforced by the autonomous Panama Canal Authority.