Jefferson penned the letter in 1787, while performing his diplomatic duties. He was a seminal figure in the crafting of the Declaration of Independence and the Constitution. He wrote most of the former and assisted James Madison with the formal construction of the latter.
William Smith maintained correspondence with Jefferson to apprise him of developments with the crafting of the Constitution. Because he was a liberal, Jefferson was committed to personal freedom more than many of the Federalist statesmen at the time.
In the letter, it is easy to sense that Jefferson is upset that pieces of the Constitution began to sway towards more centralized power in response to Shays' Rebellion.
Daniel Shays led a group of Massachusetts residents in a rebellion in which he attempted to seize the Springfield Armory and subsequently overthrow the US government.
In his letter, Jefferson refers to the need for a rebellion every twenty years. In a very specific way, Shays' Rebellion is what he was referring to. It was a healthy precipitant for a recognition of the people from those who sought to rule them.
This event was probably the single most important reason that General George Washington agreed to leave his home in Virginia and head back to New York to lead the fledgling republic into a new age of constitutional rule.
In the context of these events, consider the words of Jefferson, and think about what liberals like Jefferson might have thought about various events of our contemporary era.