Devereux was the highest ranking aristocrat who supported Parliament when the war started in 1642, and led the Parliamentary armies for three years. However, a notable lack of military success and changing political fashions resulted in his fall from grace.
As unimaginative as his father had been dashing and flamboyant, Essex first rose to prominence during the Long Parliament of 1640 when he participated in attacks on Strafford and formed a close bond with John Pym. Indeed, it was his close relationship with Pym (Essex had warned Pym of Charles' plans to arrest him in January 1642) plus his aristocrat status which resulted in his elevation to a senior military post.
During his three years in charge of the Parliamentary armies, Essex recorded no outstanding successes and was repeatedly criticised for his failure to zealously prosecute the war effort. However, as long as he enjoyed Pym's confidence, his position was secure. He performed competently at Edgehill but failed to harry the retreating Royalist army as it withdrew to Oxford. He captured Reading and relieved the siege of Gloucester, but failed to record any truly decisive victories.
Pym died in December 1643, but it was military incompetence and political moderation which contributed to Essex's downfall. Against the orders of Parliament, he pursued Royalist forces into the West Country in 1644 and was heavily defeated at the Battle of Lostwithiel. Cut off in Cornwall, Essex managed to escape with his cavalry but was forced to leave his infantry and artillery behind.
This debacle coincided with major divisions within the Parliamentary camp between the moderate Presbyterians (who favoured a negotiated end to the war) and the Independents (who favoured outright military victory). Essex was unambiguously linked to the old guard, and they were progressively outmanouevred and marginalised during the latter part of 1644. In December 1644, Parliament approved the Self-Denying Ordinance which prohibited peers and MPs from holding military commands (though an exception was made for Cromwell). Realising that his time had passed, Essex resigned his commission on 2nd April 1645, the day before the Self Denying Ordinance came into effect.
He died in 1646 and was buried at Westminster Abbey.