A variety of U.S. laws and regulations exist to monitor and controls exports from the United States. These laws and regulations are designed to comply with trade agreements, embargoes, sanctions and other political measures the U.S. has with other countries. Most importantly, the laws and regulations are designed to protect U.S. national security so the most sensitive information and technology don't get into the wrong hands.
This is where the International Traffic in Arms Regulations (ITAR) come in. ITAR is part of a web of laws and regulations prohibiting U.S. individuals and companies from engaging in business with prohibited/sanctioned countries and persons for various economic, financial, anti-terrorism and human rights issues.