Yes, it's origins are from a American WW2 poster. The suggestion of knitting comes from the subtly stylized texture of the yarn and the imperative to "purl harder," i.e. knit diligently to support the war effort. By charging the readers to "remember Pearl Harbor," and using more subtle prompts, such as the color yellow, chopsticks, and noodles, the poster is urging readers to remember who bombed Pearl Harbor - "our enemies." The V-shape created by the intersecting knitting needles suggests that victory can be achieved via the support of knitters at home who endeavor to "purl harder."

Yes. It was a poster from WW11 calling for help to knit soldiers mittens and hats. Purl is a knitting stitch. REMEMBER PEARL HARBOUR. PURL HARDER.