The other countries would team up against the American government and hold them responsible for the disaster.
And it made perfect sense why Dr. Manhattan left in the comics.
Yeah, but it's not something that Veidt could have counted on in the comics. By making him an enemy of the world, there's no reason for him to stay.
And why would the other countries team up on America? I mean, I get the general idea, but that just leads to global destruction. A common enemy provides that.
Look at the comic. A squid monster hits the US and Dr. Manhattan leaves. SO you have a weakened US with their most powerful entity gone. Why wouldn't they attack then?
See? The argument can still be made either way. The movie makes sense as much as the comic does, and I think it tied things together better.