Using the time frame 1800-1849, argue that 19th century American history can be explained by sectionalism.
There were many incidents in the time frame that you mention that show that American history was being driven by sectionalism. I will mention three of those incidents here.
First, there was the controversy that led to the Missouri Compromise in 1820. This showed that the two sections of the country already felt at that time that it was vital for them to have equal representation in the Senate.
Second, there was the Nullification Crisis of 1832 and 1833. This showed that the South felt that the North was ignoring its needs and that the South was willing to more or less break with the Union in order to get its way.
Finally, there was the Mexican-American War. To some extent, support for this war broke down along sectional lines. Many Northerners opposed the war because they thought that it was being fought solely to gain more territory for the slave states.