Well, considering that:
- Interstellar Travel would take a lot of resources
- It would also take a good deal of time.
- It would, almost certainly, NOT be by accident
- Those who are not able to travel between stars would be viewed as primates in comparison
- There's no guarentee that alien intelligence would even resemble and anthropomorphic (human-shaped) body
I'd say the only reason I'd be visiting Earth would either be to attack, to colonize, or collect resources. It would not be in my best interests to
make peaceful contact, because humans are a parasitic, primitive, poisonous species in comparison. At best, they'd be good fodder for food or slaves,
but more likely I'd realize their intellectual capacity and deem them unfit for either.
I'd also realized their limited attack range in space, but their surgical attack capability worldwide. There would be no such thing as a foolhearty
risk, if I were in command of a spacefaring vessel filled with lives depending on our mission. I'd watch and observe. If I deemed them to be a
threat, I'd either leave, or plan an attack, based upon the mission, and how critical the objective was and the relative risk vs. reward of trying to
conquer the pesky humans.
Assuming I decided not to leave, I'd scan the planet until I determined where all major concentrations of military force and leadership were.
I would learn enough of the languages to relay such commands as "surrender or we will continue" and "gather at this point"
Assuming I could discern enough of the political structure of Earth, I'd take out the U.N. during a meeting, while simultaneously taking out the
major military installations, whether through disabling or destruction. By the time the first attack wave was over, the second wave would continue on
to secondary targets, which would be all sattelites and any major ground-based centers of communication, such as Tokyo Tower, and other huge radio and
television relay stations.
My losses would be minimal, and any counterstrike would be blind, as the satellites would likely be destroyed before anyone pieced together that the
attack came from space. By that time, my artilliary craft would have moved far enough into position that orbital bombardment of every major city and
remaining military installation could begin. I'd start with the industrial cities and military sites first, so as to halt or cripple production. Next
would be the highest population sites, such as India and China, so as to reduce the fighting force available.
After that, mop-up crews would be dispatched to deal with any remaining resistance.
Estimated time from attack to mop-up, with a small fleet, about 4 hours, depending on the speed of our craft.