The first federal statutes imposing the legal obligation to pay a federal income tax were adopted by Congress in 1861 and 1862 to pay for the Civil War. This income tax was repealed in 1872, but a new income tax statute was enacted as part of the Wilson-Gorman Tariff Act in 1894.

The United States Constitution specified Congress could impose a direct tax only if it was apportioned among the states according to each state's census population. In its 1895 decision the Supreme Court held in the case of Pollock v. Farmers' Loan & Trust Co. that a tax on income from property (a tax on interest, dividends or rent) was a direct tax under the Constitution, and so had to be apportioned.

The concept of terrorism is itself controversial because it is often used by states to delegitimize political or foreign opponents, and potentially legitimize the state's own use of terror against them. A less politically and emotionally charged, and better defined, term (used not only for terrorists, and not including all those who have been described as terrorists) is violent non-state actor.

War is not necessarily considered to be the same as occupation, murder, or genocide because of the reciprocal nature of the violent struggle, and the organized nature of the units involved.

A civil war is a war between factions of citizens of one country (such as in the English Civil War). A proxy war is a war that results when two powers use third parties as substitutes for fighting each other directly.