[a] All members of the House of Commons or Lords who were also officers in the Parliamentary army or navy were required to resign one or the other, within 40 days from 3 April 1645.

At the outset of the First English Civil War, Parliament gave command of its main armies to members of the aristocracy.

[2] As the war proceeded, it was clear that Essex and Manchester were at best half-hearted in pursuing the fight against the Royalists, an attitude that became ever more apparent as the struggle became more radical.

Members of Parliament, notably Oliver Cromwell and Sir William Waller, saw the need for radical reform of the army.

At the height of this bitter controversy, Cromwell suddenly proposed to stifle all animosities by the resignation of all officers who were members of either House.

Leadership of Parliament's troops fell to Sir Thomas Fairfax, then a lieutenant general, who was among the few officers still eligible for the post.

In addition to a professional officer corps promoted on merit, it replaced the sometimes bulky local units with nationally controlled regiments, standardized training protocols, and ensured regular salary payments to the troops.

This army soon turned the war in favour of Parliament, decisively beating the Royalist forces at the battle of Naseby on 14 June 1645.