Answer:
In the 1850s, the US government shifted from a strategy of permitting slavery to extend into new areas to one of striving to limit its development. A succession of legislative initiatives, notably the Compromise of 1850 and the Kansas-Nebraska Act of 1854, reflected this shift in policy.
Previous to this policy shift, the US government had adopted a strategy of permitting slavery to expand into new areas in order to preserve a balance between free and slave states. This approach was codified in the Missouri Compromise of 1820, which placed a line across Missouri's southern frontier, with territory north of that line restricted to slavery and those south of it open.
Yet, the acquisition of additional territories like as Texas and California, as well as the ambition of certain Southern leaders to spread slavery into those territories, heightened tensions between the North and South. In an effort to calm tensions and maintain the Union, the US government began to adjust its policy toward limiting the development of slavery in the 1850s.