According to the preamble to the constitution, one purpose of the government is to keep the peace domestically. It states that the government should insure domestic tranquility, defense the citizens and promote there welfare.
The preambre of the constitution states:
"We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defense, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America."