The Heirs of C. and M. De Armas v The United States was a Supreme Court case that addressed the issue of whether or not land grants made by Spain to individuals prior to the Louisiana Purchase were still valid after the purchase had been completed. In this case, two Spanish citizens had received a grant for certain lands in Florida from Spain before it was ceded to France and then purchased by the United States in 1803. The heirs argued that their rights as owners should be respected under international law since they had acquired them before any transfer occurred between nations, but the US government contended that such rights could not stand against its own sovereignty over newly-acquired territories. After considering both sides' arguments, the court ultimately ruled in favor of The United States on grounds that no private right can exist which is inconsistent with public policy established by Congress when acquiring new territory; thus, all pre-existing titles became nullified upon acquisition and are subject only to laws enacted thereafter within those boundaries.

In the case of The Heirs of C. and M. De Armas v. The United States, the dissenting opinion argued that Congress had no authority to pass a law which would grant land titles in Florida to individuals who had been living on it prior to 1821, when Spain ceded its claim over the territory to the United States. This was because such an act would be contrary to both international law and public policy as established by treaties between nations at that time; specifically, any title granted by one nation could not be transferred or recognized by another without mutual agreement from both parties involved in said treaty. Furthermore, even if Congress did have such power under certain circumstances (which they do not), this particular instance does not meet those criteria due to lack of evidence presented regarding ownership rights prior to Spanish rule over Florida being held by either party involved in this dispute - thus making it impossible for them or anyone else claiming similar rights before 1821to receive legal recognition for their claims now after so much time has passed since then.