When Edmund's son Thomas, 2nd Earl of Lancaster, inherited the estates and title of his father-in-law Henry de Lacy, 3rd Earl of Lincoln, he became at a stroke the most powerful nobleman in England, with lands throughout the kingdom and the ability to raise vast private armies to wield power at national and local levels.

[2] This brought him—and his younger brother Henry, 3rd Earl of Lancaster—into conflict with their first cousin King Edward II, leading to Thomas's execution.

After the supporters of Henry III of England suppressed opposition from the English nobility in the Second Barons' War, Henry granted to his second son Edmund Crouchback the titles and possessions forfeited by attainder of the barons' leader, Simon de Montfort, 6th Earl of Leicester, including the Earldom of Leicester, on 26 October 1265.

[4] After initially supporting Edward, Thomas became one of the Lords Ordainers, who demanded the banishment of Piers Gaveston and the governance of the realm by a baronial council.

[9] Mortimer lost support over the Treaty of Edinburgh–Northampton that formalised Scotland's independence, and his developing power in the Welsh Marches provoked jealousy from the barons.

When Mortimer called a parliament to make his new powers and estates permanent with the title of Earl of March in 1328, Henry led the opposition and held a counter-meeting.

Edward III was able to assume control in 1330 but Henry's further influence was restricted by poor health and blindness for the last fifteen years of his life.

[15] After the outbreak of the Hundred Years' War, Henry took part in several diplomatic missions and minor campaigns and was present at the great English victory in the naval Battle of Sluys in 1340.

Lancaster was also given palatinate status for the county of Lancashire, which entailed a separate administration independent of the crown.