In 1854 the Kansas-Nebraska Act, which allowed southern planters to bring their enslaved peoples to the nation’s western territories, upended American politics, shattering the fragile consensus on slavery that had held since the drafting of the US Constitution. Historian David Brown traces the impact of the controversial legislation throughout that fateful year, which saw the creation of the Republican Party, put Lincoln on the path to the presidency, and ensured that no more compromises between the sections would be possible.