This is how Starscream forms an alliance with MECH goes in Operation: Bumblebee Part 1.
[We see MECH working on something]
Starscream: [laughs] You are the ones that turned poor Breakdown into a pitful science experiment? Didn't learn much from the expierment, did you?
[The MECH soldiers charge their weapons, ready for a fight]
Silas: Enough to know where the soft spots are, Decepticon.
Starscream: Oh please, I no longer affiliated with that entity. Besides, if I were to avenge the big lug, you would be screaming for mercy by now.
Silas: Why are you here?
Starscream: To propose an alliance. You see we have much more in common than you think. And I am not merely referring to our battle scars.
Silas: What could I possibly gain from such an arrangement?
Starscream: Insider knowledge.
[Silas gives the MECH the signal not fire and they back away]
Starscream: Wise choice. Now, a T-Cog isn't technology. It is biology. Which means it will reject any power source... except Energon.
Silas: Go on.
Starscream: Energon is the fuel, the ammunition, and the lifeblood of all Cybertronians. Whatever their affiliation. You and I both want it. And currently, I lack the means of locating it. But once I supply you with the… [scratches his finger] Particulars of its chemical makeup.
[Starscream douses some into the robot body on the wall]
Starscream: We can utilize your considerable resources to devise the means of detecting new deposits for us to share.
Silas: If the supply meets MECH's rather significant demand.
Starscream: Ahhh, intending to build an army are you?
[He turns to him]
Starscream: I am no stranger to ambition.