"In the United States, law enforcement and diplomatic instruments lead the migration and border security effort. The DHS's U.S. Customs and Border Protection (CBP) is the principal federal law enforcement agency charged with enforcing agriculture, trade, drug and immigration regulations. They operate in all three border environments -- land, sea, and air -- within North America and internationally with cooperating governments. This effort is complemented by the U.S. Department of State's (DoS) 'Border Security Program.' Through proactive efforts in its embassies and consulates, DoS has improved information sharing, connectivity, and training to identify and prevent foreigners bent on doing harm from acquiring travel documents allowing them legal entry to the United States.5 The military's mission to defend U.S. borders from aggressors is principally accomplished through terrorist deterrence, detection, and interdiction before they reach the United States. However, events since 9/11 have led to the deployment of military forces to temporarily supplement and complement the CBP. Such was the case immediately following 9/11, when 1600 National Guard troops were federalized for duty on our northern and southern borders. Again, in 2004 following the intelligence community's assessment of a heightened threat along the northern border between Vermont, New Hampshire and New York, active duty and reserve component forces were deployed along those borders in the execution of Operation Winter Freeze. That augmentation took place between the federal election day and the presidential inauguration."
Army War College: http://www.csl.army.mil
Center for Strategic Leadership Issue Paper (October 2009), v.15-09