As Hobbes has said, the US, UK, China and Nigeria are at the head of their list and will always be the first to fall when an infiltration mission is launched in that region. Afterwards, other countries will fall.
As far as I know, there is nothing you can do to prevent an infiltration mission from reaching its conclusion. Although shooting UFOs does help delay the next wave and make the defection occur later, there is nothing you can do to prevent the ultimate fall (except attack Cydonia before the country falls, I guess).