Specifically from the Constitution of the United States:
1) To Form a More Perfect Union: the government was set in place to keep the States together and form a better, unified nation
2) Establish Justice: one of the major causes of the creation of government was to establish laws and enforce them
3) Ensure Domestic Tranquility: protect American citizens from others (domestic)
4) Provide The Common Defense: another major cause for the creation of government was to establish a strong military, capable of defending the nation against all threats
5) Promote The General Welfare: enduring the nation and citizen’s overall welfare
6) Secure The Blessing of Liberty: providing and ensuring the safety of citizens basic rights; Life, Liberty and the Pursuit of Happiness