The intervention of the Elector Maurice of Saxony against Charles V., of King William against Louis XIV., in 1688, of Russia and France in the seven years' war, of Russia again between France and Austria, in 1805, and between France and Prussia, in 1806, are examples under the second head.

Most liberal-publicists consider intervention in the internal affairs of nations as indefensible; but the principle is supported by the advocates of the old monarchies of Europe.
_Wars of insurrection_ to gain or to regain liberty; as was the case with the Americans in 1776, and the modern Greeks in 1821.
_Wars of independence_ from foreign dictation and control as the wars of Poland against Russia, of the Netherlands against Spain, of France against the several coalitions of the allied powers, of the Spanish Peninsula against France and of China and India against England.

The American war of 1812 partook largely of this character, and some judicious historians have denominated it the war of Independence, as distinguished from the war of the Revolution.
_Wars of opinion_, like those which the Vendeans have sustained in support of the Bourbons, and those France has sustained against the allies, as also those of propagandism, waged against the smaller European states by the republican hordes of the French Revolution.

To this class also belong-- _Religious wars_, like those of Islamism, of the crusades, and of the Reformation.
_Wars of conquest_, like those of the Romans in Gaul, of the English in India, of the French in Egypt and Africa, and of the Russians in Circassia.
_National wars_, in which the great body of the people of a state engage, like those of the Swiss against Austria and the Duke of Burgundy, of the Catalans in 1712, of the Americans against England, of the Dutch against Phillip II., and of the Poles and Circassians against Russia.
_Civil wars_, where one portion of the state fights against the other, as the war of the Roses in England, of the league in France, of the Guelphs and Ghibelines in Italy, and of the factions in Mexico and South America.
It is not the present intention to enter into any discussion of these different kinds of war, but rather to consider the general subject, and to discuss such general principles and rules as may be applicable to all wars.
War in its most extensive sense may be regarded both as a _science_ and an _art_.

It is a science so far as it investigates general principles and institutes an analysis of military operations; and an art when considered with reference to the practical rules for conducting campaigns, sieges, battles, &c.