"rifle behind every blade of grass."

I'm going to focus on this statement in particular. That is attributedto Isoroku Yamamoto, the commander in chief of the Japanese navy. He also is quoted to have said "I fear all we have done is to awaken a sleeping giant and fill him with a terrible resolve."

No one has found any evidence that Isoroku said either of those statements.

The latter statement came from the film Tora, Tora, Tora. No one can even figure out where the rifle statement rumor even started.

However, Isoroku did give two other statements that showed he held the mentality of the second quote.

Should hostilities once break out between Japan and the United States, it is not enough that we take Guam and the Philippines, nor even Hawaii and San Francisco. To make victory certain, we would have to march into Washington and dictate the terms of peace in the White House. I wonder if our politicians, among whom armchair arguments about war are being glibly bandied about in the name of state politics, have confidence as to the final outcome and are prepared to make the necessary sacrifices.

Source: At Dawn We Slept (1981)

Here, Isoroku disagreed with the Japanese general strategy of scaring the US out of the fight by attacking it's islands. He believed that the only way to defeat the US was to go all the way to the capital and force peace. While the fear of a "rifle behind every blade of grass" may make that more difficult, the bigger problem is that the US mainland's distance from Japan and geography (between the West Coast and DC there are thousands of miles, deserts, big rivers, and two major mountain ranges). Such an invasion, if it was even possible, would cost the Japanese dearly.

In the first six to twelve months of a war with the United States and Great Britain I will run wild and win victory upon victory. But then, if the war continues after that, I have no expectation of success.

Source: Eagle Against the Sun: The American War With Japan (1985)

Basically, at the start of the war the Japanese would be able to dictate the war and gain many quick victories (which did happen, with the invasion of US held islands like the Philopines and some islands in Alaska). However, Isoroku knew Japan's resource and production problems would make victory against the US impossible in the long run.

So, the Japanese were cautious of invading the US mainland, their entire plan at Pearl Harbor was to "knock" the US out of the fight. However, it was a problem of distance, geography, and resources, rather than the fear of the second amendment.