Standish's early military career in the Low Countries is unclear. At the time, the Dutch Republic was embroiled in the Eighty Years' War with Spain. Queen Elizabeth I of England supported the Protestant Dutch Republic and sent troops to fight the Spanish in the Netherlands, as part of the Anglo-Spanish War (1585–1604). Historians are divided on Standish's role in the English military. Nathaniel Philbrick refers to him as a "mercenary", suggesting that he was a hired soldier of fortune seeking employment in the war, but Justin Winsor claims that Standish received a commission as a lieutenant in the English army and was later promoted to captain while in Holland. Historian Jeremy Bangs argued that Standish likely served under Sir Horatio Vere, the general who led the English troops in the Netherlands at this time. Vere is known to have recruited soldiers for the expedition in both Lancashire and the Isle of Man, among other places. According to historian Tudor Jenks, Standish came to the Netherlands around 1603 and may have seen service during the siege of Sluis in 1604, which involved Vere's English troops. The subsequent Treaty of London (1604) ended English involvement in the war; if Standish was a mercenary he might have continued to serve with the Dutch until the Twelve Years' Truce brought fighting in the region to a halt in 1609.