You should have read the whole paragraph, not just the "external" threat part.
We recently saw in the case of the terrorist attack on Mumbai that terrorists may enter a country covertly by ocean routes, landing in small craft and then assembling to wreak havoc. That we have a lawfully armed populace adds a measure of security for all of us and makes it less likely that a band of terrorists could make headway in an attack on any community before more professional forces arrived. Second, the right to bear arms is a protection against the possibility that even our own government could degenerate into tyranny, and though this may seem unlikely, this possibility should be guarded against with individual diligence
The second amendment was indeed intended to assure and armed citizenry, which could be used to repeal external threats, but which could also be used to contain an out of control government. That last part clearly echos that liberals' liberal:
Certainly one of the chief guarantees of freedom under any government, no matter how popular and respected, is the right of the citizens to bear arms. . . . [T]he right of the citizens to bear arms is just one guarantee against arbitrary government and one more safeguard against a tyranny which now appears remote in America, but which historically has proved to be always possible.
-Senator Hubert H. Humphrey, "Know Your Lawmakers", _Guns_, Feb. 1960
Ninth Circuit Rules 2nd Amendment Incorporated to States Monday April 20, 2009, 5:46 pm EDT
BELLEVUE, Wash.--(BUSINESS WIRE)--The Second Amendment Foundation today applauded the U.S. Ninth Circuit Court of Appeals in San Francisco for ruling that the Second Amendment is incorporated against the states and local governments.
The majority opinion was written by Judge Diarmuid F. O'Scannlain, with a concurring opinion from Judge Ronald M. Gould, who wrote, “The right to bear arms is a bulwark against external invasion…That we have a lawfully armed populace adds a measure of security for all of us and makes it less likely that a band of terrorists could make headway in an attack on any community before more professional forces arrived.”
Although the court found against the plaintiffs in the case of Nordyke v. King – Russell and Sallie Nordyke, operators of a gun show in Alameda County, CA – the court acknowledged that its earlier position that the Second Amendment protected only a collective right of states has been overruled by the Supreme Court’s 2008 historic ruling in District of Columbia v. Dick Anthony Heller. That was the case in which the high court ruled that the Second Amendment protects an individual civil right to keep and bear arms.
“This is a great victory for advancement of the fundamental individual right of American citizens to own firearms,” said SAF founder Alan Gottlieb. “The Ninth Circuit panel has acknowledged that the Heller ruling abrogated its earlier position on the Second Amendment, and it further clarified that the Second Amendment is incorporated to the states through the Fourteenth Amendment through the due process clause.”
SAF attorney Alan Gura, who successfully argued the Heller case before the Supreme Court in March 2008, filed an amicus brief in the Nordyke case. The Nordykes sued when Alameda County banned gun shows at the county fairgrounds by making it illegal to bring or possess firearms or ammunition on county property.
“The Heller ruling in 2008 was the first critical step toward full restoration of the individual citizen’s right to keep and bear arms to its rightful position as a cornerstone of the Bill of Rights,” Gottlieb observed. “This victory in the Ninth Circuit not only reinforces the Heller ruling, it expands upon it.”
The Second Amendment Foundation (www.saf.org) is the nation’s oldest and largest tax-exempt education, research, publishing and legal action group focusing on the Constitutional right and heritage to privately own and possess firearms. Founded in 1974, The Foundation has grown to more than 600,000 members and supporters and conducts many programs designed to better inform the public about the consequences of gun control.
Please show me where, in this document, the main purpose of the Second Amendment is covered??