I forgot the exact episode number but I know when it
happened in the show as I love both Treize and Une. *sigh*

It's towards the beginning...

This is after Une sets all the missiles at Victoria on
self-destruct so that the blast would be big enough to
destroy the Gundams (they would not have the time to
fly off the perimeter -even Wing in Bird Mode-). 
Noin stops Une, and that scene is afterwards, when 
Treize explains to Une that destroying nature for a 
few Gundams is wrong. And there he tells her
to be more elegant. ^^;

Zoi

*sigh* How I wish I had caught this update on time. 
Those Treize cels were simply gorgeous!