I'd like to have a go at answering this if I may,
yes, I believe Mankind would co-operate together, (my enemies enemy is my friend), an old Adage, but true.
However, after the threat had been defeated, mankind would start fighting amongst themselves again. We just can't, and never will, live peaceably together, and I sincerely hope we never discover intelligent life on another planet. because we'd fight them too.
As an aside, the film you mention, is just another slant on H.G. Wells 'War of the Worlds'