Bannockburn and aftermath
Beaumont accompanied Edward II on his campaign into Scotland in 1314, and subsequently fought at the Battle of Bannockburn. On the first day of the battle Henry was in one of the two cavalry forces alongside Robert Clifford and Sir Thomas de Grey of Heaton, father of the chronicler Thomas Grey whose account of events follows;
Robert Lord de Clifford and Henry de Beaumont, with three hundred men-at-arms, made a circuit upon the other side of the wood towards the castle, keeping the open ground. Thomas Randolph, 1st Earl of Moray, King Robert's nephew, who was the leader of the Scottish advanced guard, hearing that his uncle had repulsed the advanced guard of the English on the other side of the wood, thought that he must have his share, and issuing from the wood with his division marched across the open ground towards the two afore-named lords.
Sir Henry de Beaumont called to his men: "Let us wait a little; let them come on; give them room".
"Sir," said Sir Thomas Gray, "I doubt that whatever you give them now, they will have all too soon".
"Very well" exclaimed the said Henry, "if you are afraid, be off".
"Sir," answered the said Thomas, "it is not from fear that I shall fly this day."
So saying, he spurred in between Beaumont and Sir William Deyncourt and charged into the thick of the enemy. William was killed, Thomas was taken prisoner, his horse being killed on the pikes, and he himself carried off with the Scots on foot when they marched off, having utterly routed the squadron of the said two lords. Some of the English fled to the castle, others to the king's army, which having already left the road through the wood had debouched upon a plain near the water of Forth beyond Bannockburn, an evil, deep, wet marsh, where the said English army unharnessed and remained all night, having sadly lost confidence and being too much disaffected by the events of the day.
In the November after Bannockburn, Beaumont and his wife were those affected by the sentence of forfeiture passed by the Scottish parliament against all those with land and title in Scotland who continued to fight with the English. This group of nobles came to be known as "The Disinherited", and included David III Strathbogie, Beaumont's future son-in-law who claimed the Earldom of Atholl. Henceforth, Beaumont would dedicate a large part of his career to recovering his wife's Scottish lands.
For some time, Beaumont remained loyal to Edward II, fighting on his side at the Battle of Boroughbridge in 1322. However, when Edward II entered into truce negotiations with the Scots in May 1323, Beaumont, hitherto a close associate of the king, argued against any agreement which disregarded the claims of the disinherited, for whom he had become the leading spokesman. Edward and Beaumont quarraled when the latter allegedly refused to swear an oath to Edward and Hugh Despenser the Younger "to live and die with them." Beaumont was briefly imprisoned for contempt and disobedience at the Privy Council (of which he was a member), after which he retired from Court to continue his intrigues in exile, eventually joining forces with Edward's estranged wife, Queen Isabella, and her lover Roger Mortimer, 1st Earl of March.
Following the deposition of Edward II in 1327, Beaumont was initially well rewarded by the new regime of Isabella and Mortimer. However, following a failed Scottish campaign in 1328, and anxious to break the deadlock in the north, Isabella and Mortimer persuaded Parliament to accept the terms of the Treaty of Northampton, which ignored, once again, the claims of the disinherited. Many of the senior nobility repudiated what they considered to be a shameful peace; and when Henry, 3rd Earl of Lancaster rose in revolt in late 1328 he was joined by Henry Beaumont and other members of the Disinherited, including Thomas Wake, Henry Ferrers, Thomas Rosselin and David Strathbogie, the latter now married to Beaumont's daughter, Katherine. This was the nucleus of the party soon to be prominent supporters of Edward Balliol, the son of the former King John Balliol. The rising was short-lived; and when Lancaster submitted in January 1329, Wake and Strathbogie also made their peace. Not so Henry Beaumont, who was specifically excluded from pardon, going into exile to plot Mortimer's downfall.
When Edmund of Woodstock, 1st Earl of Kent was arrested in March 1330 and charged with conspiring to restore Edward II, whom he believed was still alive, he alleged at his trial that Beaumont had met him in Paris and told him that his plot would be supported from Scotland by the armed intervention of Donald, Earl of Mar, a personal friend of the ex-king. Kent was executed and Beaumont would never be allowed to return to England while Mortimer and Isabella held on to power.