Complex sentence: On the English mainland, the Lancastrians were quick to exploit the Yorkist flight; the Earl of Wiltshire was appointed Lieutenant of Ireland and Henry Beaufort, 3rd Duke of Somerset became Captain of Calais.
Split into simpler sentences: On the English mainland, the Lancastrians were quick to exploit the Yorkist flight. The Earl of Wiltshire was appointed Lieutenant of Ireland and the Duke of Somerset became Captain of Calais.