Red on top, white on the bottom, nothing else on the flag. That’s the entire design shared by Indonesia and Monaco, and it’s about as simple as a national flag gets, which is exactly why the two are so easy to mix up.
Monaco‘s flag is the older of the pair. The red and white have been the heraldic colors of the House of Grimaldi, Monaco’s ruling family, since at least the 14th century, though the modern flag wasn’t formally adopted until 1881, under Prince Charles III. Indonesia‘s flag looks identical but has a completely separate origin, tracing back to the red and white banners of the Majapahit Empire, a powerful maritime kingdom that ruled much of Southeast Asia between the 13th and 16th centuries. When Indonesia declared independence in 1945, it revived those old colors as the new national flag.
Since neither country borrowed from the other, the resemblance is pure coincidence, helped along by the fact that red and white happen to be two of the most common colors in flag design worldwide. That didn’t stop it from becoming a genuine diplomatic issue. In 1952, Monaco formally asked Indonesia to change its flag to avoid confusion. Indonesia said no, pointing out that its colors predated Monaco’s independence recognition by centuries in their own right. The dispute was eventually settled not by redesigning either flag, but by adjusting the proportions: Monaco’s flag is a 4:5 ratio, giving it a noticeably squarer shape, while Indonesia’s is the more standard 2:3, making it longer and slimmer.
It’s a subtle fix, and one that only really helps if you have both flags in front of you to compare. On a still day with no wind, or shrunk down to a small icon on a screen, the two are still nearly impossible to tell apart. Poland, worth mentioning here too, has the same two colors flipped, white over red, and has occasionally been displayed upside down at international events, momentarily turning it into Indonesia’s flag by accident.