I’d say provide for the general defense because one state can not protect the whole country
Well one reason could be that nobody is at a higher status than another so there's little chance of a dictatorship or anything. A different reason is a balance of power leaves no desire to gain more from unfairness.
"The Emancipation Proclamation was an order by U.S. President Abraham Lincoln to free slaves in 10 states."(simple.wikipedia.com).
The nickname is Shot heard round the world.