COEHOORN, MENNO, Baron van (1641-1704), Dutch soldier and military engineer, of Swedish extraction, was born at Leeuwarden in Friesland. He received an excellent military and general education, and at the age of sixteen became a captain in the Dutch army. He took part in the defence of Maastricht in 1673 and in the siege of Grave in the same year, where the small mortars (called coehorns) invented by him caused the French garrison considerable trouble (Seydel, Nachrichten über Festungskriege, Leipzig, 1818). He was made a colonel for his gallant conduct at the battle of Seneff (1674), and was present also at the battles of Cassel (1677) and Saint Denis (1678).
The circumstances of the time and the country turned Coehoorn's attention to the art of fortification, and the events of the late war showed him that existing methods could no longer be relied upon. His first published work, Versterckinge de Vijfhoeks met alle syne Buytenwerken (Leeuwarden, 1682), at once aroused attention, and involved the author in a lively controversy with a rival engineer, Louys Paan (Leeuwarden, 1682, 1683; copies are in the library of the Dutch ministry of war). The military authorities were much interested in this, and entrusted Coehoorn with the reconstruction of several fortresses in the Netherlands. This task he continued throughout his career; and his experience in the work made him the worthy rival of his great contemporary Vauban. He formulated his ideas a little later in his chief work, Nieuwe Vestingbouw op en natte of lage horizont, etc. (Leeuwarden, 1685), in which he laid down three "systems," the characteristic feature of which was the multiplicity and great saliency of the works, which were calculated and in principle are still eminently suited for flat and almost marshy sites such as those of the Low Countries. He borrowed many of the details from the works of his Dutch predecessor Freytag, of Albrecht Dürer, and of the German engineer Speckle, and in general he aimed rather at the adaptation of his principles to the requirements of individual sites than at producing a geometrically and theoretically perfect fortress; and throughout his career he never hesitated to depart from his own rules in dealing with exceptional cases, such as that of Groningen. Subsequent editions of Nieuwe Vestingbouw appeared in Dutch (1702, and frequently afterwards), English (London, 1705), French (Wesel, 1705), and German (Düsseldorf, 1709).
From 1688 to the treaty of Ryswick Coehoorn served as a brigadier. At the battle of Fleurus he greatly distinguished himself, and in 1692 he defended Namur, a fortress of his own creation. Namur was taken by Vauban; but the Dutch engineer had his revenge three years later, when the place, on which in the meantime Vauban had lavished his skill, fell to his attack. Coehoorn became lieutenant-general and inspector-general of the Netherlands fortresses, and the high-German peoples as well as his own countrymen honoured him. He commanded a corps in the army of the duke of Marlborough from 1701 to 1703, and in the constant siege warfare of these campaigns in the Low Countries his technical skill was of the highest value. The swift reduction of the fortress of Bonn and the siege of Huy in 1703 were his crowning successes. At the opening of his following campaign he was on his way to confer with Marlborough when he died of apoplexy at Wijkel on the 17th of March 1704.
His "first system" was applied to numerous places in Holland, notably Nijmwegen, Breda and Bergen-op-Zoom. Mannheim in Germany was also fortified in this way, while the "secondsystem" was applied to Belgrade and Temesvár in eastern Europe.
His son, Gosewijn Theodor van Coehoorn, wrote his life (re-edited Syperstein, Leeuwarden, 1860). See also v. Zastrow, Geschichte der beständigen Befestigung (Leipzig, 1828); von Brese-Winiari, Uber Entstehen und Wesen der neueren Befestigungsmethode (1844); Cosseran de Villenoisy, Essai historique sur la fortification (1869); Mandar, Architecture des forteresses (1801); Krayenhoff, Verhandeling over de erste versterkingsmanier van Coehoorn (Hague, 1823); Bosscha, Nederlandsche heldend te Land (Amsterdam, 1838); Dewez, Histoire de Belgique (Brussels, 1823); Ypey, Narratio de rebus gestis Mennonis Cohorni (1771); Hennert, Dissertation sur la fortification permanente (1795); Böhms, Gründliche Anleitung zur Kriegsbaukunst (1776); Axiomatas of allgemeene bekentnisse over de Vestinghbouw door Menno Baron van Coehoorn, Uytgewerkt door E. W. Berg (MS. in Dutch Ministry of War); Bousmard, Essai général de fortification (1797); also the article Fortification and Siegecraft.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)