DHS's mission is to lead the unified national effort to secure the United States by preventing and deterring terrorist attacks and protecting against and responding to threats and hazards to the nation. As part of that mission, DHS is responsible for ensuring that the nation's borders are safe and secure, that they welcome lawful immigrants and visitors, and that they promote the free flow of commerce. Within the department, CBP is responsible for customs, immigration, and agricultural processing at ports of entry. 4 ICE is responsible for the investigation and enforcement of border control, customs, and immigration laws.