It's unnatural and unhealthy for a nation to be engaged in global crusades for some principle or idea while neglecting the needs of its own people.
Once imbued with the idea of a mission, a great nation easily assumes that it has the means as well as the duty to do God's work.
Power tends to confuse itself with virtue, and a great nation is peculiarly susceptible to the idea that its power is a sign of God's favor.