How to Subjugate your neighbours

[Saxon]

Laying waste to your enemies is, naturally, my goal and the only reason I ask this question!

Seriously, sometimes I would like the extra resources, but am too stretched to crush a small, usually one system, race. Once I beat the big enemy down, I can turn to knocking enough points off another player to get them to surrender.

Does one need to have peaceful relations with the other nation before they will accept the treaty or can they be at war with you? In other words, is beating them down to a sixth or an eighth of your size and then demanding a treaty possible?