With publication of Federalist Paper #14, Objections to the Proposed Constitution From Extent of Territory Answered, James Madison brings the argument of union vs disunion to a close. He explains clearly the difference between a democracy and a republic.
To the critics who claim that a republic must be confined to a small territory, Madison writes that they confuse a republic with a democracy. The difference, according to Madison, is that in a democracy the people meet and exercise the government in person, whereas in a republic the people govern the country through their elected representatives. Because a republic has representatives, it can extend over a large region.