New Model Army was formed from the existing Roundhead forces on February 15, 1645 onwards when Oliver Cromwell passed the Self-denying Ordinance to remove the former leadership of the Roundhead forces. It was led by Thomas Fairfax and at first Cromwell was only in the charge of the cavalry. It finally came into being in April.
New Model Army consisted of 22.000 soldiers, including 11 regiments of cavalry (6600 men), 12 regiments of infantry (14400 men) and 1 regiment of 1000 dragoons. Soldiers were conscripted from all over the country and older regiments were merged to join the army.
Leadership and promotions were meant to be based on proficiency instead of social standing or wealth. However, Cromwell also preferred soldiers that were devoted Puritans like himself and some of them sung psalms prior to battle. He also instituted standard daily pay (8 cents for infantry, 2 shillings for cavalry) and guarantee of food, clothing and other provisions. Cavalrymen had to supply their own horses.
Cromwell merged men from multiple regiments into a single one and provisioned them with red uniforms to replace their former regiment colors. "Soldier's catechism" dictated new regulations and drill procedures.
Prince Rupert, one of the King's followers, nicknamed them as Ironsides. However, this was more due to their ability to cut through opposing forces than their armor, as has been sometimes claimed; their armor was composed of leather jerkins.
Tactics of the New Model Army were based on fast hit-an-run attacks against the flanks of the enemy. Frontal attack would have meant exposing them to the Royalist artillery. Cromwell specifically forbade them to pursue a fleeing enemy and hold the battlefield.
New Model Army won important victories at Naseby (when they were first used) and Preston. After the end of the civil war, they fought in Scotland, Ireland, Flanders and Spain. After the campaigns in Ireland in 1640's some of them retired to settle in what is now Northern Ireland.
In November 1647 the Army held an important debate about political, constitutional and religious issues in St Mary's Church, Putney.
During the World War One, Douglas Haig re-introduced the term for his units of the British Expeditionary Force. This army was soon decimated at the Somme, and the term became something of a joke.
See also: New Model Army (band)