Uncle Sam represents the government. Liberty replaced Columbia as the symbol of America because of the Statue of Liberty.
You are correct. Uncle Sam was first used around the early 1800s, during the War of 1812. However, he was not particularly well used until the early rumblings of state secessions. He was brought back like a legacy character in order to represent the Northern government being the big grownups to mock the unruly states acting like children. During this time, Columbia was ALSO used by northern (and southern) sympathizer cartoonists of anti-slavery, anti-racism movements. She went from representing Freedom, Liberty, American expansion and excellence to being shown giving protection to jews and chinese and blacks from the evil tyranny of the racist southern business heads who wanted to keep them building railroads.
Columbia, prior to this, was the catch all. America WAS it's government until the heavy separation of the two into almost two different things, which continues to this day.
And replacing Columbia with Liberty was silly, since she already represents this, and more. Don't you take my Columbia from me, she's America.
EDIT: because I'm retard and can't spell
EDIT 2: Because I'm curious and you got me digging, it looks like "Uncle Sam" as we know him now, didn't exist until 1917. His look and representation as we understand it now was based on Lincoln and a few others. He had earlier precursors, like Brother Jonathan. And a few mentions and art unlike what we envision now. But the origins of Uncle Sam are murky until 1917 solidified the character and what he represented. Some say it was based on a meat packer named Samuel Wilson. Others say that the term originated from in jokes in the military. Like people on merchant vessels asking what the US stamped on goods meant, and soldiers joking that it came from Uncle Sam.
