Copenhagenization refers to the practice of confiscating the warships of a defeated enemy. It first occurred when the British fleet under Admiral Gambier defeated the Dano-Norwegian fleet in the Second Battle of Copenhagen in 1807.
After the British Navy confiscated the Dano-Norwegian navy, the practice of confiscating all (or most) of the ships of a defeated enemy became more common and would be expressed by the term Copenhagenize. In 1830 American author Richard Emmons published an epic poem on the late war of 1812, the The fredoniad, or Independence preserved in which he wrote of the merits of the independents merits:
Aw'd by the naval sceptre of the king—
Our fleet would Copenhagenize each town,
And with the torch burn every hamlet down.
The term would later be used by Justin Winsor in his Narrative and critical history of America (1888) where he described the outfitting of independent vessels to warfare was done somewhat covertly, in order to avoid the vessels might "be Copenhagenized at once by the invincible British Navy" at the outbreak of hostilities and also in the 1881 Political Science, Political Economy, and the Political History of the United States, John J. Lalor, editor:
But, even when [the embargo] was repealed in 1809, the belief that Great Britain would "Copenhagenize" any American navy which might be formed was sufficient to deter the democratic leaders from anything bolder than non-intercourse laws, until the idea of invading Canada took root and blossomed into a declaration of war.
In 1940 after the fall of France, the British destroyed the French navy of neutral Vichy stationed in the ports of Oran and Dakar with the Attack on Mers-el-Kébir, fearing that the French ships would fall into German hands. 
- Emmons, Richard (1830). The fredoniad, or Independence preserved. Philadelphia. p. 35
- Winsor, Justin (1884). Narrative and critical history of America 7. Boston: Houghton, Mifflin and Company. pp. 273–274
- Cyclopædia of Political Science, Political Economy, and the Political History of the United States by the Best American and European Writers. Published: New York: Maynard, Merrill, and Co., 1899. Online version, II.18.13 and II.18.26.
- Azar, Gat (2006). War In Human Civilization. New York: Oxford University Press Inc. p. 610