I can think of many examples of how diversity has led to obstacles that America has overcome. However, I am unaware of how diversity, in and of itself, has been a strength.
In the beginning of American History there were lots of clashes between different denominations of Christians. America needed to unite to gain independence and freedom from the British. Our forefathers united around their common (not diverse) belief that our freedoms and inalienable rights are granted by God, not government. We overcame all obstacles with our common beliefs as expressed in the Declaration of Independence and the US constitution.
America's slogan that is stamped on all coins states: "E Pluribus Unum" which means "out of many, one". This means that we get our strength from the unique American culture, not from diversity.
The cultural differences between the North and South led to the bloodiest war that this nation has ever endured. It cost 750,000 lives to overcome this obstacle and to reunite America.
Diversity of race led to Jim Crow laws, discrimination and cruelty against blacks by whites. We overcame this obstacle with the civil rights movement.
In today's America, businesses and government spend billions of dollars on diversity sensitivity programs. These programs are intended to overcome obstacles caused by racial, religious, sexual and cultural diversity.
Congress has enacted hate-crime laws in an effort to reduce crimes that are caused by diverse groups clashing with each-other.
The previous examples prove that overcoming obstacles caused by diversity has cost America hundreds of thousands of lives and billions of dollars. It has all been done for just causes and America could not have survived if we had not overcome these obstacles. However, these Obstacles are the result of diversity. If there was no diversity, these obstacles would not have existed. Diversity has been an obstacle, not a strength.
On the other hand I can think of no example of how diversity, in and of itself, has benefited America besides adding variety to food, music, and the arts from diverse cultures. Of course people from diverse racial, cultural religious and sexual backgrounds have contributed greatly to America, but this is not due to their diversity, this is due to their individual achievements. Humans of all races, religions, cultures and sexes have the same potential for greatness. No one group has more potential than another. Mixing groups together does not add to, nor diminish each individuals potential for greatness. Humans of all races, religions and cultures are equally endowed by God.
It is horrible when diverse groups can't live, work, play and worship together. Many obstacles had to be overcome to achieve tolerance in America. Tolerance is a very, very good thing, but it is only required BECAUSE of diversity. Diversity, in and of itself is not a virtue. Tolerance IS a virtue.
Most liberals preach that "diversity is our strength". That statement is a lie. Tolerance is a strength, diversity is an obstacle. Many cultures have achieved greatness without diversity. Japan, Finland and Switzerland are three examples.
Diversity is a challenge that America has overcome. America has not become great because of diversity, we have become great in spite of diversity.