The most important purpose of government as detailed in the Preamble to the U.S. Constitution is:
To establish justice.
Explanation:
The other purposes of government are secondary to the establishment of justice. The enthronement of justice ensures the realization of the other purposes. Without justice, the government will not be able to achieve any domestic tranquility. Without justice, the government will not be able to realize the common defense of all. Without justice, the government cannot promote the general welfare of the citizenry. Without justice, the government cannot secure the blessings of liberty. Therefore, justice is the fulcrum for the sustenance of the existence of every government. Otherwise, the government simply becomes a pack of cards, without meaning and order.