Well their attitude towards me is murderous and mine is the same towards them, the treaty was first signed when the Phong attacked me, and I was actually outclassed by both them and the Solars....as I said it was out of my survival though the partnership treaty was actually the first thing they proposed and I took it
Besides that my economy is in shambles at the moment and I need their resources and above all else the Icarans are survivors and if it takes allying with a hated enemy for the time being then ally they will.
[edit]: Oh and remember these are Icarans we are talking about when I say "trusted" ally I simply mean one that is not making any immedietly threatening moves against me.