Sure it is. But if torture was limited to only cases as clear-cut as the one you propose, it would be virtually non-existent.

For such rare, extreme cases, it's better to keep it illegal, and hope an interrogator will risk jail to save the million people (and possibly be freed by jury nullification), than (effectively) legalizing it, and have its use creep to anyone merely suspected of terrorism (as defined by, from the point of view of the torture victims, an invading force), by an organization unaccountable even to citizens of its own country.