The term 'WATCH ON THE ___' is completed by the crossword solution RHINE, which is a major river in Europe. The Rhine is 1, 233 miles long and flows through Switzerland, Liechtenstein, Austria, Germany, France, and the Netherlands. It is one of the most important rivers in Europe and has been an important trade route since ancient times. The phrase 'watch on the Rhine' is often used to refer to vigilance and preparedness for potential conflict.