Quote Originally Posted by John_Douglas View Post
I thought you could say вступился meaning to step in and break up a fight - but maybe I'm confused.
It is possible in a specific context only, when intervening person takes one specified side against the other.

Multilex says:
intervene ["Intq'vi:n] v
...
2. вмешиваться, вступаться, посредничать
to intervene to settle a quarrel — вмешаться, чтобы положить конец ссоре
to intervene in a dispute — вмешаться в спор
...



Anyway, I think it's wrong to translate "intervene" and "interfere" with the same word.
I believe in Russian my translations are OK. It is not always possible to translate nuances like positive or negative color of each word.

Посредничать (to mediate) is a good equivalent of the "intervene" as you mean it but it has not a perfective couple so you have to make complicated constructions (as Lampada pointed out) like "Министр США сыграл роль посредника в разрешении кризиса" or "Министр США предложил свои услуги посредника в разрешении кризиса".

Note that in Russian it is grammatically impossible to inter(vene) the fight without being accepted by each of the sides and still be good and fair.

Maybe you could use суется for "interfere".
In my opinion it is an equivalent of "вмешивается" but strongly colloquial and with even more negative bias.