Coehorn, Or Cohorn, Menno Van, baron, a Dutch general and engineer, born in Friesland in 1641 (according to some in 1632), died at the Hague, March 17, 1704. A captain at the age of 16, he distinguished himself at the siege of Maestricht, and at the battles of Senef, Cas-sel, St. Denis, and Fleurus. During the intervals of active duty he devoted much attention to the subject of fortification, with the view of equalizing the chances between besiegers and besieged, the new system of his contemporary Vauban having given great advantages to the latter. While a young man he gained a name as an engineer, and by the time he had reached middle life was recognized as the best officer of that arm in the Dutch service. The prince of Orange promised him a colonelcy, but as he was remiss in fulfilling the pledge, Coehorn retired in disgust, with the intention of offering his services to the French. His wife and eight children, however, were arrested by order of the prince as hostages for his return, which quickly brought him back, when he received the promised rank, and was afterward appointed successively general of artillery, director general of fortifications, and governor of Flanders. His whole life was spent in connection with the defences of the Low Countries. At the siege of Grave, in 1674, he invented and for the first time made use of the small mortars called cohorns, for throwing grenades, and in the succeeding year elicited the applause of Vauban by successfully crossing the Maas, and carrying a bastion which was considered as protected by the river.

After the peace of Nimeguen (1678) he was employed in strengthening various already strong places. Nime-guen, Breda, Bergen-op-Zoom, and other fortresses, attest the value of his system. The last named place he considered his masterpiece, but it was taken after a long siege in 1747 by Marshal de Lowendal. During the campaigns from 1683 to 1691 he was in active service. The siege of Namur in 1692 gave him an opportunity to test his system against that of Vauban, for these two great engineers were there opposed to each other, Coehorn in defending a work which he had constructed to protect the citadel, and Vauban in attempting to reduce it. Coehorn made an obstinate defence, but, being dangerously wounded, was compelled to surrender to his rival. He was afterward engaged at the attacks on Trarbach, Limburg, and Liege, and in 1695 aided in retaking Namur. In the war of the Spanish succession he besieged successively Venloo, Stephensworth, Roeremond, and Liege; and in 1703 he took Bonn, on the Rhine, after three days' cannonade of heavy artillery aided by a fire of grenades from 500 cohorns.

Next he passed into Flanders, where he gained several successes over the French, and subsequently directed the siege of Huy. This was his last service, for he died soon afterward of apoplexy, while waiting a conference with the duke of Marlborough on the plan of a new campaign. Coehorn's greatest work, Nieuwe Vestingbouw (fol., Leeuwarden, 1685), was translated into several foreign languages. His plans are mostly adapted to the Dutch fortresses, or to those which are similarly situated on ground but a few feet above water level. Wherever it was practicable, he encircled his works with two ditches; the outermost full of water, the inner dry, and usually of the width of about 125 ft., serving as a place d'armes for the besieged, and in some cases for detachments of cavalry. The theory of his system, both of attack and defence, was the superiority of a combined mass over isolated fire. Professionally, Coehorn was accused of wasteful expenditure of life, in which respect he contrasted unfavorably with Vauban, who was sparing of men. He refused inducements offered by several foreign governments. Charles II. of England knighted him.

He was buried at Wijkel, near Sneek, in Friesland, and a monument was dedicated there to his memory; His biography was written by his son Theodorus (new ed., by Sypestion, 1860). For his system of fortifications, see Zastrow, Geschichte der Befestigung (3d ed., 1854).