In "Redemption", the legally elected klingon government (supported by most of the population) asked for federation help against, essentially, terrorists.
The federation refused to interfere, because stopping terrorists is an 'internal matter'.
If this 'internal matter' was beyond the federation's authority, why should romulan help for the klingons be anything the federation can interfere with?
Now the federation can't interfere in internal politics, but has free reign to interfere in external politics (that do not involve it?)? What's the justification for this division?
And, of course, an romulan-klingon alliance would be disastruous for the federation.
The federation's refusal to interfere was suicidal.
Picard saved the day only because the romulans acted equally opaque:
One wonders why, after Picard&co found the romulans, Sela didn't say:
"You found me. Big deal. I just returned from your ship, Picard. You know nothing you didn't already know.
This changes nothing - this is not hide and seek; I have no reason to return just becase you see me.
If you have the balls, open fire.
If not, get out of my way. I have a war to win. And, when I'm done, I'll come with my new klingon pals and start taking federation worlds apart"