The U.S. made many concessions to slave holders to keep the Union together. For example, the U.S. army put down a slave revolt in New Orleans in 1811. Slave rebellions were seen as threatening to the social order and property. Later, John Brown, who raided the National Arsenal in Harper's Ferry, Virginia, with the hopes of starting a widespread slave revolt, was executed by the state of Virginia with the blessing of the federal government. 

In addition, the government made political concessions to slave owners to keep the Union together. For example, the Compromise of 1850 included a stronger Fugitive Slave Law in return for allowing the state of California admission into the Union as a free state. The law was hated among abolitionists, as it facilitated the recapture of slaves in the north. The Supreme Court also played a role in upholding slavery. In the 1857 case Dred Scott v. Sandford, the Supreme Court held that slaves could not sue for their freedom because they were property. 

Even after Lincoln was elected in 1860, he tried to mollify the south until hostilities increased and he needed the support of abolitionists. In 1862, Congress passed a Confiscation Act, allowing the Union to free slaves in areas they conquered, but the Union generals did not do so and Lincoln did not insist that they do so. Even the Emancipation Proclamation of 1863 only freed slaves in the south and was therefore largely symbolic, as the states where it freed slaves were not under federal control at the time. The Union fought the Civil War to keep the country together, not to abolish slavery, though it eventually did so.

Zinn makes the point that slavery had become a practical reality in the new nation and thus impossible to repudiate.  The opening of Zinn's writing demonstrates this:

The United States government's support of slavery was based on an overpowering practicality. In 1790, a thousand tons of cotton were being produced every year in the South. By 1860, it was a million tons. In the same period, 500,000 slaves grew to 4 million.

Such a reality made it fundamentally impossible for American capital to develop without the presence of slaves.  It was in this light where Zinn asserts that slavery could not be demolished as it had become such an ingrained part of the economic culture and future of the United States.  Zinn suggests that legislation like the Fugitive Slave Act of 1850 helped to continue the practice of slavery as it lessened restrictions on slaveowners who wished to recapture escaped slaves.  At the same time, the lack of recognition of rights for people of color made it easier to continue the practice of slavery.  This denial of voice happened on social, economic, and political levels.  Zinn makes the argument that as long as the American government denied voice to people of color and needed slavery to continue its economic advancement towards prosperity and dominance, slavery was always going to be supported in a manner that would prevent its dismantling.