Who, specifically, did Yanagida mean by "half the world"? In episode 4, Yanagida tells Itami that the suits back in Nagata want to know whether the Special Region is worth turning half the world against Japan. 

Who, specifically, consitutes "half the world" here? As of this point in the anime, we haven't been told much about the political situation surrounding the Gate. Is there a particular bloc of nations that opposes Japan's activity in the Special Region? Or is this more of a figurative "lots of people will be angry with us if we colonize the Special Region" sort of thing?
In the manga, in chapter 7, Yanagida mentions:


  Is there something here that's worth turning America, China, and Russia... half of the world into our enemies?


It's been mentioned that (emphasis mine): 


  The major powers, China, the European Union (EU), India, Japan, Russia, and the United States, contain just over half the world's people and account for 75 percent of global GDP and 80 percent of global defense spending.


So he's likely to mean most of developed countries that are world powers. Most notably America, China, and Russia.