Spooner, like Warren, was an activist. He challenged the US Postal Service monopoly by starting his own mail company, forcing the postage rates down but eventually having to go out of business due to continuous legal harassment. Spooner is probably most famous for his abolitionist activities. He not only offered free legal services to fugitive slaves and those who assisted them, but wrote what was considered the definitive legal analysis: The Unconstitutionality of Slavery. This paper was extremely constroversial even among his fellow abolitionists. William Lloyd Garrison's position was that the Constitution condoned slavery, calling it "a covenant with death and an agreement with hell." Garrison, more anarchist than Spooner at this time, favored secession by northern states so as to avoid having to enforce the Fugitive Slave Act. To Garrison, one should not support a state which endorses and enforces slavery. In later years, Spooner would come around to the pure anarchist position that constitutions were merely statist excuses for engaging in "legalized" crime.

The practical difficulty with our government has been, that most of those who have administered it, have taken it for granted that the Constitution, as it is written, was a thing of no importance; that it neither said what it meant, nor meant what it said; that it was gotten up by swindlers, (as many of its authors doubtless were,) who said a great many good things, which they did not mean, and meant a great many bad things, which they dared not say; that these men, under the false pretence of a government resting on the consent of the whole people, designed to entrap them into a government of a part; who should be powerful and fraudulent enough to cheat the weaker portion out of all the good things that were said, but not meant, and subject them to all the bad things that were meant, but not said. ...

The result - and a natural one - has been that we have had governments, State and national, devoted to nearly every grade and species of crime that governments have ever practised upon their victims; and these crimes have culminated in a war that has cost a million of lives; a war carried on, upon one side, for chattel slavery, and on the other for political slavery; upon neither for liberty, justice, or truth. - Lysander Spooner, No Treason (1867-1870)