Charles Boxer says that Michiel Doorn was actually a Spaniard, Miguel de Orna (or d'Orna), born at Pamplona in the Navarra province in Northern Spain. He typically used a Dutch name, however.
On 18 February 1639, 12 Dunkirk warships, 3 pinnaces, and 5 fluits sailed for La Coruña under command of Michiel Doorn. The fluits carried 2000 Walloon soldiers for the Armada. The Dutch were warned ahead of time, and Tromp set sail to intercept them with 12 ships. A hard-fought battle ensued. The result was that Doorn was forced to return to Dunkirk. The Dutch were heavily damaged and had to withdraw.
In July 1639, when Tromp had gone to the Shetlands, We saw how the Vice-Admiral de With was sent to the Channel with seven ships. He joined a squadron there under commandeur Ham, so his ship strength had grown to eleven ships. Shortly after the meeting, eleven sails came into view. It was the Dunkirk fleet, which came from the Shetlands. The commander was Michiel Doorn, who the Spaniards called De Horna. His vice-admiral was Mathijs Rombout. They had two prizes with them, of the ships found at the Shetlands, and a later prize, a Straatsvaarder of the Rotterdam burgemeester Coulster.
On 27 August 1639, when the Spanish Armada sailed from La Coruña, Michiel Doorn was on board Admiral Oquendo's flagship Santiago to advise him. Doorn's Vice-Admiral, Mathijs Rombout, was on board the Nuestra Senora de Monteagudo.Sources:
- Dr. M.G. De Boer, Tromp en de Duinkerkers, 1949.
- Charles R. Boxer, The Journal of Maarten Harpertszoon Tromp Anno 1639, 1930.