In peace or at peace?

Hi!

Is the use of "at or "in" in the following sentence acceptable?

"Japan has been in/at peace with the rest of the world since 1945."

Is there any difference in meaning?

"At peace" seems to mean only "without war" because I belive that we can't say "he is sleeping at peace" but "in peace."

Is there any guideline when to use "at" or "in" in connection with peace?

Thanks for your help.