9, titled "The Union as a Safeguard Against Domestic Faction and Insurrection", is a political essay by Alexander Hamilton and the ninth of The Federalist Papers.

It was first published in the New York Daily Advertiser and the Independent Journal on November 21, 1787, under the pseudonym used for all Federalist Papers, Publius.

The essay argues that large republics can achieve stability, and that they do not inevitably lead to tyranny as his opponents believe.

Hamilton responded with other writings of Montesquieu, presenting the argument that a larger republic could exist as a confederation of states like the one proposed in the constitution.

Publius argues that the American states must unite to avoid the failures of the Ancient Greek and Roman republics.

He cites separation of powers, checks and balances, judicial service during good behavior, and representative democracy.

He counters that Montesquieu's vision of a small republic is not applicable to the American states, as splitting them into smaller commonwealths would cause them to war with one another or necessitate that they are governed under monarchy.

Publius quotes another argument of Montesquieu to demonstrate the philosopher's support for a confederate republic to accommodate a larger state.

He cited The Spirit of Law by the French philosopher Montesquieu, which argued that large nations could not survive as republics.

[8]: 5  He cited modern understanding of political science as an advantage the United States had over the ancient republics,[5]: 70  including separation of powers and representative government.

[3]: 126  Unlike most of The Federalist Papers that take a tone of warning, his thoughts on scientific progress present an optimism derived from Enlightenment philosophy.

Listening to Madison's speeches, Hamilton wrote into his notes: "the way to prevent a majority from having an interest to oppress the minority is to enlarge the sphere".

They believed that a unification of the states would create a nation too large to be a republic, citing the tyranny that developed as Greece and Rome expanded.

Hamilton also presented Montesquieu's suggestion that Lycia was an ideal of confederation with its cities of varying size and strength.

[5]: 69  One of the foremost purposes of the proposed government cited by Hamilton was to prevent societal instability caused by a constant military threat between the states.

In the philosophy of Adam Smith, the preservation of stability in this fashion directly benefits everyone in society by allowing a setting for the production of economic value.

[9]: 39 Other federalists participated in the Montesquieu debate separately from The Federalist Papers: Americanus disagreed that European philosophies applied to the circumstances of the United States, and A Citizen of America wrote that the ancient republics lacked the core values of republicanism held by Americans.

9 in Heath v. Alabama (1985) to demonstrate specific aspects of sovereignty that states are expected to have, though this was not a significant influence in her overall argument.