I'm trying to configure a Security Policy to only allow US-region IP addresses to hit our network. I added as a first rule to allow any untrust us region to destination untrust US region. I am not sure if this is correct. maybe I'm allowing all traffic within US, that probably is not supposed to be allowed.
You could setup an entry that lists US as the source with the negate-source option enabled and set the application and service to any and the action to deny. This will drop any and all traffic that doesn't originate from the US, while allowing the rest of your rulebase to actually handle traffic.