China wasn't an "enemy", the Japanese had already conquered large swathes of it. The US and Britain were.
And yes, you can write "pure" yamatokotoba if you try hard enough (see eg. Shinto prayers), but the end result is as contrived as trying to write English without Latin, Greek or French loans.
Uncleftish Beholding (1989) is a short text written by Poul Anderson. It is written using almost exclusively words of Germanic origin, and was intended to illustrate what the English language might look like if it had not received its considerable number of loanwords from other languages, particularly Latin, Greek and French.
Historically, the Japanese people have been very willing to borrow words from other languages.