If an asteroid is large enough to wipe out the third largest country on the planet, then its going to have a SIGNIFICANT effect on everyone else.
If it takes out a medium sized city, sure its going to be a big deal but its not going to be a massive world wide problem. It will alter the climate...but maybe only as much as a large scale volcanoe eruption.
I figure, if we know about it now...if we determine that Earth may be in danger...we have enough time to build something that will help us out. Alter its course, make it smaller by throwing nukes at it, or build a kinetic weapon and try and do both.