The second amendment guarantees states the right to form armed militias (or to put it in modern terms, armed police forces).
This is thoroughly false. "[T]he right of the people to keep and bear arms shall not be infringed." The prefatory clause about the militia explains the motivation but does not recognize or protect anything on its own. "The people" have a First Amendment right to peaceably assemble; a Second Amendment right to keep and bear arms; a Fourth Amendment right to be secure in their persons, houses, papers and effects against unreasonable searches; and retain rights not granted to the federal government or reserved to the states under the Tenth Amendment. "The people" have individual rights, not collective ones.
And a militia is a military force, not armed police. In the 18th Century, the militia was called into service to fight Indian tribes, French forces in the colonies, and various rebellions. Armed police forces would not have filled the need even if they existed at the time (the first modern US police department was created in 1838, and the next was 1845).