Complex sentence: A Scottish army was assembled under the leadership of John Earl of Buchan and Archibald Earl of Wigtown and from late 1419 to 1421 the Scottish army became the mainstay of the Dauphin's defence of the lower Loire valley.
Split into simpler sentences: A Scottish army was assembled under the leadership of John Earl of Buchan and Archibald Earl of Wigtown. From late 1419 to 1421 the Scottish army became the mainstay of the Dauphin's defence of the lower Loire valley.