I'm not entirely sure about international law in the early 19th Century except insofar as there were agreements implied and inferred by the powers involved, and that was about as far as it went in those day, I think.

I'm also not sure where you can point anything Napoleon did that was illegal, politically, diplomatically or militarily in the wider scheme of things. What he did do was threaten neighbouring regimes perceptions of their security, such that war was inevitable.

I have yet to make up my mind about this; was he incredibly stupid or did he deliberately set out to antagonise his neighbours?

JC