How did slavery conflict (go against) the principles of the declaration of independence?

According to the Declaration of Independence, every human being living in the United States has the right to be free and decide its own ways in life, and than on the other hand we have the slavery, where people were enslaved, were either slaves for all of their lives or occasionally they were able to buy out their freedom, and they did not decided for their own lives but their owners. These two are the two total contrasting opposites that occurred in the American society, and they show how it is possible that from one negative extreme, the country can be reformed and functioning in another positive extreme.

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explanation:

nothing certain is known about what happened, but it seems that the country declined gradually, perhaps due to bad government, or, more likely, due to a series of bad harvests, which resulted in widespread and recurrent famine.