(_c_) =1815 to date.= The Peace of Westphalia (1648), the Peace of Utrecht (1713), and the Treaty of Vienna (1815) are the three celebrated cases of combined action of modern European powers. The "balance of power" idea had gradually been supplemented by "the concert of the powers" idea, which would not merely maintain the relative _status quo_ of "the balance," but might enter upon a positive policy of concerted action. The "Holy Alliance" of 1815, to promote "Justice, Christian Charity, and Peace,"[36] was first broken by its originators. There was a strong feeling that the principles of international law should be followed, however, and this, the "Declaration of the Five Cabinets," Nov. 15, 1818, distinctly avowed in "their invariable resolution, never to depart, either among themselves, or in their relations with other states, from the strictest observation of the principles of the Rights of Nations."[37] The attempt to extend the principle of intervention in favor of maintaining the various sovereigns on their thrones, and in suppression of internal revolutionary disturbances by foreign force was made in the "Circular of the Three Powers," Dec 8, 1820.[38] Under many forms intervention has been one of the great questions of the nineteenth century, and the growing proximity and multiplication of relations of states during the century has added many complications.[39] The Grecian War of Independence (1821-1829) brought the new principle of pacific blockade (1827), and at its conclusion the powers guaranteed the sovereignty of Greece. The subjects of right of search, foreign enlistment, Monroe Doctrine, freedom of commerce and navigation, expatriation, extradition, neutralized territory, ship canals, consular rights, neutral rights and duties, arbitration, reciprocity, mixed courts, international postage, weights and measures, trade-marks and copyright, rules of war, sub-marine cables, and sphere of influence, which have come to the front during the nineteenth century, indicate in a measure the subject-matter of international negotiation. Throughout the period since 1815 the tendency has been rather to regard what _is_ the international practice.
Among the writers upon subjects connected with international law before the days of Grotius the most prominent are Victoria ( -1550?), Ayala (1548-1584), Suarez (1548-1617), and Gentilis (1551-1611). While in many respects their contributions to the science were valuable, the work of Grotius stands out preëminent among all the early writers.