They’re ceremonial now, but historically drums (and trumpets and flags and such) were how you gave orders during a battle.
Edit: Wow. Did not anticipate that much attention to an offhand ELI5 answer. Here’s a link to some excellent options if you’d like to help Ukraine, be it humanitarian support or otherwise. Every bit helps: https://linktr.ee/RazomForUkraine
Edit v2.0: Shoutout to all the commenters for elaborating on my brief response. Made for some interesting reading
Latest Answers