People hung the national flag for the Japanese Emperor’s visit.
The one true sovereign is the Japanese Emperor, aka the Emperor of Japan. All other emperors aren't simply 帝 on its own.
To remember the reading, think about how the Japanese Emperor, can always pull out his "me card" (みかど) when he's in trouble. It's kind of like how a "trump card" is the one that's above all the others, but with the emperor he's always up there, so it's a "me card" to him. And who's gonna argue with the emperor? Not me.