Aware of hostility toward Britain from the forced merger of the five New England colonies by Governor Andros, Section 3 provides, however, that no new state can be created by altering the borders of one or more existing states without the consent of that state or states. In the Compromise of 1820, Massachusetts agreed to relinquish its Maine territory to become a free state balancing Missouri which was admitted as a slave state. Section 4 obligates the U.S. government to defend a state against invasion and also provides the basis for state governors to ask the federal government to send in troops to put down riots, stop looting, etc. It is this provision that was the basis for mobilizing the National Guard to quell riots in various cities during the 1960s. The basis for sending in active duty troops is Article II, under the president's duty to enforce the law, which includes implementing federal court decisions.
Article V provides for amending the Constitution.
It generates four possibilities. So far the latter has not been used. When an amendment is proposed, the proposing body specifies which of these options is to be used for ratifying the amendment. So far, conventions haven't been used. Two methods of proposing and two of ratifying yield a total of four combinations as methods for amendment.