Picture this. It's 1949 and you're reading the newly published Bartholomew and the Oobleck. You flip the pages and see green goop falling from the sky. Twenty-first century you might be thinking acid-rain-ozone-global-warming-world-is-ending (and we'll get to that), but in 1949, you're probably thinking duck and cover. The experts agree:
"Bartholomew […] rework[s] the wartime realities of assault from the air and territorial occupation into backdrops of children's tales." (Source.)
That's right. That green goop is quite a bit scarier when we think of it in terms of war. Assaults from above were a major reality in 1949, and the oobleck is just another manifestation of that.