A government regulation is that the answer!!
The accusations made against King George III had a direct effect on what the founding fathers considered important to include in the declaration of independence, which ended up setting the basis of the new American government to be guided by the later written Constitution. Due to the political climate at the time, the declaration of Independence was created as a sort of a justification for the necessity of separation from England; based on the concerns of the colonist about the way King George III was ruling at the time, characterized by practices such as abuse of power, tyranny, and exclusion of the colonist in government.
Ratification of the 13th Amendment “HAD NO INFLUENCE ON” the voting strength of the Southern States
I think the constitution prohibits the enactment of ex post facto.