While I was playing OpenXcom, a few infiltration main waves appeared (Terror ship, supply ship, battleship), and of course, I shot down all infiltration craft quickly before the second battleship should have arrived (second battleship=pact is sealed). One of them, over Brazil, had what I thought to be the second pact sealing battleship appear in Earth's atmosphere over USA but was shot down. I'm surprised that despite these waves, no country has signed a pact with the aliens by the end of the game. Ufopaedia says that once the main infiltration wave begins the pact process, it cannot be stopped, only delayed by shooting down craft before the second battleship arrives. That's not what happened when I played the game.
Perhaps the pact is sealed if the second battleship is not shot down before reaching the infiltrated country.
