For 200 years the U.S. was a white, christian, country. Now whites are becoming a minority. Granted they make up the largest minority, so far, but they are now, or soon will be, no longer the majority. It seems obvious to me that racial fractures will be a threat to the unity of the country.
If the U.S. tries to maintain the white power structure in the face of this growing cultural diversity this will no doubt continue to fuel racial turmoil leading to national instability. An unstable world power is not a good thing.
On the other hand, carelessly embracing and encouraging cultural diversity without a strong sense of national unity could also weaken the power structure and lead to instability.
The U.S. is going through some growing pains right now. How do we maintain a sense of unity in all this diversity?