You could argue that Isengard was Italy. A small place with a lot of ambition, willing to deal with the devil in an effort to expand it's power and once glorious rule.
Morder would be Germany. Kept at bay and resting for many years, gathering it's strength until a time it was confident it could crush all opposition.
Gondor would be Britain. The one great power with enough resolve to stand up and defy the evil on it's doorstep. A shining beacon for people to rally to and a pillar of strength despite constant attacks.
Rohan would be Russia. They tried to stay out of the fight, but when it was revealed they were tricked, they rose up against their common foe and smote them. (lol ..smote)
I personally didn't see the Christian message in the book, but maybe because I wasn't looking for it. The Chronicles of Narnia however, was quite obvious.