A black, massive Hong Kong-registered ship inched its way through the churning waters of the Panama Canal — guided by tugboats stern and aft, and sweating Panamanians hoisting yellow ropes to latch onto the ship's higher levels.

Newly inaugurated U.S. President Donald Trump is pushing to "take back" the Panama Canal, the world's second busiest interoceanic waterway.

The critical waterway has a complex history. Why does Trump want it back? And what does China have to do with it?

Whether the U.S. or Panama own the Panama Canal, one of the largest routes for ship traffic in the world still faces horrible drought that will slow traffic through it, or shutter in completely from time to time.