modern armies vs G1 Decepticons

[RecklessPrudence]

Yeah, about our only hope is we get insanely lucky and catch just a few of them when they're low on Energon, hit them with enough firepower to take them out, and then hope that one of the - quite a few - selfish opportunists takes advantage. I'm assuming that we wouldn't have enough intel to know to approach Starscream with a deal?

Alternatively, hope that the Autobots come as a package deal with the Decepticons, that they realise that if the 'cons are allowed to get this much Energon they're in even more trouble than they traditionally are, and step in. Hopefully, we'd be smart enough to realise that if we fueled the 'bots enough they'd be able to help a lot more than otherwise.

So... depend on massive luck, or cultivate alien allies who, as far as we-in-scenario know, could be as bad as the giant alien robots currently rampaging across the Earth.

[NovaSaber]

Well given that Galvatron killed Starscream with what looks easily less than subnuclear explosion (I am assuming Cybertron has an atmosphere somehow since humans can survive there without suits), I think we might be able to win if we use "John Sheriden" tactics of luring them into a city and detonating a nuke or several nukes by remote control.

Actually, Starscream's death wasn't really an explosion of any kind; he just kind of crumbled; Galvatron's cannon mode may cause some kind of chain reaction, like a phaser.
I don't think Galvatron's cannon mode was necessarily subnuclear. In Five Faces of Darkness, he destroyed two planetoids with it (one of them with a single shot).