I'm not going to to say this is an endorsement. Some people will be offended at the idea, while others will think I'm engaging in a sort of national or cultural favoritism by even mentioning it.
I've long had a sort of gut feeling that writing came to Japan via Korea. But you don't see scholars on either side (meaning in Japan or Korea) – at least not those writing for Westerners – who would suggest this. Both sides prefer to downplay whatever cultural linkages might exist. But there are many.
So, I spend a lot of time reading Language Log – a blog on specifically linguistic topics. Today there was an entry about a Japanese "kanji of the year" that included – in a sort of parenthetical digression – the following claim (attributed to someone named Bob Ramsey):
You may know this, but in the Three Kingdoms period people on the Korean peninsula also used this unwieldy device [i.e. the way that Japanese uses kanji to represent native, multisyllabic words, which in "three kingdom" times was also done in Korea but later passed out of favor], called hun by them, to write native words. But then, Chinese character readings were completely standardized by the powerful monarch King Kyongdok in the Unified Silla period, and kun (or hun) readings largely disappeared from use thereafter. Not to put too fine a point on it, but it seems pretty clear that the early Japanese learned this and most other ways of writing from people from the Korean peninsula, Paekcheans probably, but Sillans might also have played a role in the transmission of scribal methods. [emphasis added]
Why am I mentioning this? Because I've thought this for a long time, but this is the first scholarly article [err.. vaguely scholarly, anyway] that I've run across that supports this idea. So I'm annotating it here for my own future reference, I guess.