Did a Japanese general really advise against invading the United States because "There’s a rifle behind every blade of grass"? If so, was this in reference to high rates of gun ownership, or America's industrial might?

There is always more to say, but u/kieslowskifan addresses the same supposed quote in this older post, pointing out that – given both very hazy provenance and a better understanding of the context – "gun advocates' use of the "blade of grass" letter [is] somewhat obscene." You might like to review that response while you wait for fresh answers to your query.