What is your definition of legitimate? Just because you don't like it doesn't make it illegitimate.
There is no such thing as Voluntarism. The State rules with impunity and no such idea existed prior to the U.S. coming into existence, that the consent of the governed Is necessary to govern although the will of the People is a hard thing to ignore when staring down a million pitchforks.
The Legitimacy of the US government does not rest in our consent but in its ability to defend itself, the fealty of its soldiers to the fiction of the rule of law rather than a Monarch and its recognition of and by other nations who do business with it as a sovereign entity. Otherwise the Native Americans have the legitimacy of government as the original occupiers. But the world's other recognized sovereigns - recognize the US Republic as the Legitimate Sovereign Jurisdiction over the lands it claims.
Research "Conquest" in Black's Law and International Law.