[The United States Defence Department is to create a new Northern Command aimed at defending America's home soil.
Defence officials say it is one of the biggest revamps of military command structures for nearly 40 years.
In remarks quoted by the French news agency AFP on Wednesday, US Defence Secretary Donald Rumsfeld called the development a "historic first".
He said NorthCom's commander would be responsible for land, air and sea defence, leading US defence forces operating within the United States in support of the civil authorities, AFP reported.
The changes are clearly a response to the 11 September terror attacks on New York and Washington, which the US Government blames on Osama Bin Laden's al-Qaeda militants.
But Pentagon officials in Washington said the restructuring also reflected global changes since the end of the Cold War.
The changes are expected to take effect on 1 October.
New role for army
Mr Rumsfeld said earlier that the new command was aimed at better meeting "the challenges of the 21st century".
Correspondents say the new command signals a recognition that the military would be called to play a decisive internal security role in the event of an attack on the United States involving weapons of mass destruction.
The US military has previously been barred from engaging in domestic law enforcement under the 1878 Military Law, passed in the aftermath of the Civil War.
NorthCom will be responsible for the continental United States and Alaska, as well as Canada, Mexico and portions of the Caribbean, said General Richard Myers, chairman of the US Joint Chiefs of Staff.
