Fairfax is best remembered as the man who commanded the New Model Army at the Battle of Naseby in June 1645. Whereas, Charles' army was lead by Prince Rupert who was the foremost Royalist military commander in the English Civil War. Prince Rupert was very much a cavalry soldier and the Royalists may have lost the war a lot sooner had it not been for his military ability. Both Parliament and the Royalists had armies with little or no military experience, although Fairfax and Prince Rupert could be classed as professional soldiers.
Until Parliament brought in a new creation called the New Model Army which was a military unit that was used to transform the English Civil War and was based on a person's ability rather than their position within society, It was also better trained and well armed, making them more formidable. The first proper use of this army was at the battle of Naseby. This was Parliaments' turning point and after that King Charles was never able to raise an army strong enough to win.
Some historians believe that this was the main reason for Parliament winning the civil war. It could be argued, however that one of the other reasons cited for Charles losing the war was the lack of money. This was because the nobles and landowners who had sided with him didn't have any money to give him. With the support of wealthy merchants and traders, the parliamentarians had steady flow of money.
Something Charles lacked and because of the lack of money, the King couldn't buy new men and supplies for his army; therefore the army was smaller and less equipped. One of the biggest mistakes that Charles made was leaving London at the beginning of the civil war. Consequently, Parliament took control and Charles was not able to gain power again, although he tried. This was a big setback as it is hard to rule the country without controlling its' capital.
Ultimately, the Royalist forces were extinguished, they had run out of money, they had lost control of the capital and their barely trained, small army meant that they had lost the upper hand that they were dealt many times. Parliament however, was a stronger force and Parliament’s control of the seas and richer areas of England, as well as their creation of a powerful, trained and disciplined New Model Army, meant that they had more power in the end.