As soon as humans started fighting each other they used music to do it. Music as psychological warfare goes back to the Old Testament, when Joshua’s trumpets caused the walls of Jericho to crumble. No record of the effect on the troops, however.
Music and war have always been close partners. Chanting and rhythmic clashing of weapons and shields to inspire themselves and intimidate their opponents was SOP (Standing Operational Procedure) since the cave men started fighting.
It was the Romans who discovered that soldiers marched with more spirit when marching in cadence with drums or music; and modern armies still use music to augment training and bolster morale. “Over There” was the theme song for American troops in WWI, and in WWII, the favorite song for both Allied and German troops was Marlene Dietrich’s Lili Marlene.
Even today, armies train, march and often fight with musical accompaniment. The iconic sequence in the film Apocalypse Now, when American Air Cavalry helicopters attacked a Viet Cong village with “The Ride of the Valkyries” playing from loudspeakers, was not just Hollywood hype. Near the end of World War II the Germans played “The Ride of the Valkyries” to inspire their troops to continue fighting.
During the Korean War, the Chinese played funeral dirges over loudspeakers to demoralize American and Allied troops; and in at least one instance, the Chinese played Hank William’s “Your Cheatin’ Heart” over loudspeakers, presumably to make American troops homesick.
Andrew Jackson’s troops In the Mexican War marched to drum rolls written by Beethoven, and before the final assault on the Alamo, Mexican General Santa Anna ordered his band to play the El Degüello, a tune used to signify “no quarter”. The literal translation of El Degüello is “slit-throat” — playing it meant no surrender and no mercy.