Answer:
The United States used a combination of diplomacy and proxy conflict to secure access to the isthmus of Panama.
One of the main diplomatic efforts undertaken by the United States was the negotiation of the Hay-Bunau-Varilla Treaty with Panama in 1903. This treaty granted the United States the right to build and operate a canal across the isthmus, as well as to maintain a zone of influence around the canal.
In addition to diplomatic efforts, the United States also used proxy conflict to secure access to the isthmus. This included supporting and arming rebels in Panama who were seeking independence from Colombia, which at the time controlled the isthmus.
These diplomatic and proxy efforts ultimately allowed the United States to gain control over the isthmus and to build the Panama Canal, which significantly improved its ability to trade and project power in the region.