Eaton at his base of operations at Alexandria, Egypt, recruited Some 500 Arab mercenaries (composed of Egyptian and Berber infantry). and 50 Greek cannoneers.
In 1804, Eaton was given the commission of a US Navy Lieutenant. He had been a Captain, in the Legion of the United States 1792-1803; however, it is not clear if he still retained this commission after 1803. As well, he had been a Sergeant, in the Continental Army, 1780-1783. In a contract agreement (between the US and its Allies, in the war against Tripoli), forwarded to Secretary of State Madison, Eaton was designated "General and Commander in Chief" of the land forces that were to be used to carry out the operation.
Eaton, could have worn a US Navy Lieutenant’s uniform. As well, he could have worn his Captain (Legion of the United States) uniform as well. It is known that Eaton, did dress in a Berber costume: This would have had the typical tall wicker hat, covered with a large keffiyeh, with long silk covered, cotton stuffed sausage coiled around the waist, as well as the head (creating the famous large Ottoman turbans), used to protect against bladed weapons.
The US Marines rank system, was largely unchanged from the Later Revolutionary War Era/1780: U.S. Army Rank Insignia:
First-Lieutenant Presley O’Bannon (who commanded to US Marines), is usually shown in his post-1805 Dress Uniform, of a Major (based on the portrait of him); whereas at Derne he was still wearing the pre-1805 US Marine uniform.
The US Marines (from USS Argus), wore a variety of uniform styles, in 1805 . They wore the US Army's standard white linen shell jacket, in the summer. They also wore the US Army's standard high-collar (Regency Styled) 1798-1804 jacket (blue with red facings). The high-collar could also fold down. The US Marines 'shoulder wings/straps' button was placed low on the collar, rather that the shoulder itself. There were two versions: Red wing; Blue wing with red piping. The early 1794-1804 US Army jackets had fold-up cuffs buttoned to the sleeve; and later versions had a plane cuff, sewn flat on the jacket. The jacket lapels could also button across.
Fighting the Ottoman Corsairs, and on the march to Derne, the US Marines adopted Ottoman items of dress:
 Edwin North McClellan. Uniforms of the American Marines 1775 to 1829. History and Museums Division. HQ., USMC. Washington, D.C., 1982.
 The Vinkhuizjen Collection contains an illustrations which shows an Austrian cuirassier, who is "equipped with the lobster-tailed pot helmet as late as the 1780s, long after its use had died out elsewhere, when campaigning against the Ottoman Turks." (John Mollo, 'Military fashion: A Comparative History of the Uniforms of the Great Armies from the 17th century to the First World War, Barrie and Jenkins, 1972). However, these were not modern helmets for the period, the Austrians raided their State Imperial Armouries, for surviving 16th/17th century helmets in the rush to equip their cavalry in the 1780s. The source of the threat was the Ottoman's use of high-quality heavy bladed Ottoman curved swords which were generally superior to European swords, as well as the use of battle axes in close-combat. This necessitated the need for helmeted protection. This background, tends to explain the US, ‘The Company of Military Historians’, produced 'Picture 194' - BOARDING PARTY, U.S. NAVY, IN WAR WITH ALGIERS (1814-1815). This shows, the boarding party wearing standard US Cavalry helmets. These are fitted with fur-trim added to the chin straps to simulate lion-beards (said to have been worn to frighten and confused their opponents).
 A common Ottoman warfare practice wss use of warriors, the ‘‘Deli’’ (Daredevil or literally ‘‘crazy’’). Warriors, like the Delis were recent converts to Islam (usually from Bosnian, Serb, and Croat origins) and were fanatically dedicated to wage war against infidels. They wore exaggerated and wild costumes as uniforms, which were a mixture of furs and feathers of animals of prey. Their weapons also looked terrifying with exaggerated features and accessories. This figure is the same as that of the "Serdengectiler" (head givers: meaning "the one who has already given his head to enemy", a man who does not care if he lived or died). These warriors were also ecounted on Ottoman ships.
 More likely, the US Navy used boiled-leather (and iron reinforced) US Army helmets; which were current in North American armies in the 1780s-1820s, by cavalry, and light infantry, as well as the artillery, given the preference for hand-to-hand fighting with steel bladed tomahawks, and equally heavy bladed curved swords. These could be configured with bearskin sausage crests, common in the US Army at the time. Painted with the letters 'US' (in common use in the period on headgear). For added protection, these sailors/marines would have taken into use, an Ottoman item of dress - specifically made to protect against bladed weapons. This was, the use of long silk covered, cotton stuffed sausage coiled around the waist, as well as the head (creating the famous large Ottoman turbans).
Right - Extracted from the Vinkhuizjen Collection illustrations which shows an Ottoman-Egyptian soldier from 1805. Eaton at Alexandria, Egypt, recruited 500 Arab mercenaries. In 1805, Ottoman territorial soldiers, throughout the Empire and its Dominions, wore similar clothing. Wearing European –styled long waist coats, and long coats. Combined with turbaned red hats . All had near identical weapons and equipment, to that of the Janissary Corps in this period. They carried heavy calibre muskets/match-locks, which did not fit bayonets. Wore cross straps with 'black powder charge bottles’ strung from these. Powder horns were hung from the waist sashes. They were individually armed with a heavy bladed scimitars. They had identical waist sashes to that of the Janissary Corps, at this time .
  Long silk covered, cotton stuffed sausage coiled around the waist, as well as the head (creating the famous large Ottoman turbans), used to protect against bladed weapons.
Right - An 1805 Ottoman sailor Illustrated in Karargah Basimevi. (1997) Tarihten Punumuze Deniz Kuvvetleri Personel Kiyafetlerinin Gecirdigi Asamalar. [Ottoman Turkish Navy Organization and Uniforms from 1363 till 1989]. Dz.K.K.ligi Karargah Basimevi: Ankara.
Eaton at his base of operations at Alexandria, Egypt, recruited Greek mercenaries. At the battle of Derne, the US Marines were brigaded with '50 Greek cannoneers' with a cannon, sent ashore from the USS Argus - a 34 gun Brig (mostly armed with 24- and 32-pounder carronades), as well as two 12-pounder guns (likely the one sent ashore), and two 18-pounder guns. It is likely that these Greek mercenaries/cannoneers were themselves Ottoman sailors who had hire themselves-out .
 The bulk of the Ottoman Imperial fleet were manned by Greeks in this period.