Perhaps read the Preamble to the Constitution...
"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."
Our common defense is provided by the U.S. Armed Forces on the international level, your state's militia or National Guard on the national level, your state's law enforcement or state troopers, and your local law enforcement agency. Some aspects of the judicial system could also fall under this, such as the district attorney’s role in murder cases. Through the department of defense, our taxes pay for the Army, Navy, Air Force and Marines. Then there's the Coast Guard and the National Guard - they also defend us. One could also make a case that the Center for Disease Control defends us from dangerous diseases, and the National Highway Traffic Safety Administration defends us from dangerous vehicles on our roads.