Wdid settlement of the west increased tension between the north and south

As the United States of America expanded west, new states were to be created. At the creation of a state before the ratification of the 13th amendment, a referendum would be held to determine whether or not the state would embrace slavery.
 This caused tension between the North (where slavery was illegal) and the South (where slavery was legal), as both regions feared that the sudden addition of pro-slavery or anti-slavery states respectively would lead to an unfavourable power imbalance.
 To counteract the creation of slave states, Northerners poured in to the western territories so that they could vote down any potential slavery referendum. Southerners reacted by pouring in in similar numbers to counteract the Northerners.
 These two groups often clashed violently in the western territories, a phenomenon known today as "Bleeding Kansas." These outbursts of violence increased tension between the North and the South, and caused many to believe that the only way to settle the slavery debate was through all out war.

(d) it was complex and capable of highly organized efforts.