COEHOORN, MENNO, BARON VAN (1641-1704), Dutch soldier and military engineer, of Swedish extraction, was born at Leeuwarden in Friesland, and became a captain in the Dutch army. In 1673 he took part in the defence of Maastricht and in the siege of Grave, where the small mortars (called coehorns) he invented troubled the French garrison. He was made a colonel for his conduct at the battle of Seneff (1674), and was also at the battles of Cassel (167 7) and Saint Denis (1678) . He was an innovator in fortification, and his first published work, Versterckinge de Vi j f hoeks met alle syne Buytenwerken (Leeu warden, 1682) caused a controversy with Louys Paan (Leeu warden, 1682, 1683) ; Coehoorn was entrusted with the reconstruc tion of several fortresses in the Netherlands, and became the worthy rival of his great contemporary Vauban. In his chief work, Nieuwe Vestingbouw op en natte of lage horizont, etc. (Leeuwarden, 1685) , he laid down three "systems," the feature of which was the multiplicity and great saliency of the works, eminently suited for flat sites such as those of the Low Countries. He borrowed many details from his Dutch predecessor Freytag, or from Albrecht Darer, and Speckle, and studied the individual case, not theoretical perfection; e.g., at Groningen.
From 1688 to 1697 Coehoorn was a brigadier. At Fleurus he distinguished himself. In 1692 he lost Namur, a fortress of his own, to Vauban, but retook it in 1695. Coehoorn became lieu tenant-general and inspector-general of the Netherlands fortresses, and the high-German peoples as well as the Dutch honoured him. He commanded a corps in Marlborough's army from 1701 to 1703, and in the constant siege warfare his skill was of the highest value. The swift reduction of Bonn and the siege of Huy in 1703 were his crowning successes. He died of apoplexy at Wijkel on March 17, 1704.
His "first system" was applied to Nijmwegen, Breda, Bergen op-Zoom, and Mannheim, his second to Belgrade and Temesvar among other places. His son, Gosewijn Theodor van Coehoorn, wrote his life (re-edited Syperstein, Leeuwarden, 186o). His Nieuwe Vestingbouw was translated into English by T. Savery ; see also the article FORTIFICATION AND SIEGECRAFT and the bibliographies under that heading.