In just three short lines, this poem describes the return of an old bad temper first thing in the morning. The speaker doesn't announce the anger or explain it. Instead, it simply shows up, like a guest nobody invited, as the title suggests.
The key move is the comparison between two very different things: a warhead and a moth. A warhead is a weapon built to destroy, while a moth is small, soft and easily ignored. Putting them side by side captures something familiar about anger. It can arrive quietly and almost gently, fluttering in, and still carry real force. The word "old" suggests this isn't a new feeling but a habit the speaker knows well and may have hoped was gone.
The mood is wary and a little weary. There's a sense of recognition, even dry humor, but also unease. The speaker seems to watch the temper approach rather than fully control it, which makes the moment feel both small and quietly dangerous.