Leslie had a long and successful career as a soldier: attaining rank of a Dutch captain, a Swedish field marshal and, in Scotland, the title Earl of Leven. From 1605-7 he served under Sir Horace Vere in the Netherlands before transferring to the Swedish army in 1608. His career in Sweden was distinguished and he was knighted by King Gustavus Adolphus in 1626. In 1638 he returned to Scotland following the emerging crisis surrounding the signing of the Covenant. Taking command of the army of the Covenant, Leslie led the forces in the ensuing wars. He was around seventy when the third civil war broke out and tried to resign as commander-in-chief, but the Scottish parliament refused to accept it and he was required to maintain strategic command. He finally retired to his Fife estates in 1654.