> Yes, but in each of those fights, humanity's response was pretty limited by fear of collateral damage or lack of manpower on the scene.
> 
> If it came to a real war of all of humanity vs alien threat... You'd be looking at hundreds of thousands of infantry pounding those little heat guns into Megatron. I don't think he would enjoy that.

Sure, by numbers. But then you factor in the Decepticon army. It's still there, on the moon, just waiting to be activated. The story states that Megatron didn't use them only because he was so focused on the Allspark. Plus, there's bots like Soundwave. He's capable of hacking into, monitoring, and censoring practically all communication on Earth and the surrounding solar system.

And yes, nukes are scary. But compare that to a weapon that devours suns? Or can open a portal through space and time?