Lancaster and Edward have been picking at each other for years. In 1322, things finally came to a head, and rebellion was out in the open. Lancaster must have had a fighting chance, but it all goes to show that the reign of Edward is essentially about a struggle between mediocrities.
The first was Bartholomew of Badlesmere. Isabella gave Edward a causus bellum when refused entry from Badlesmere's wife to Leeds Castle (brilliant place to visit, by the way). Edward moved to take Leeds, while Mortimer, Hereford and Badlesmere dithered, and Lancaster ordered them to leave Badlesmere to his fate - he hated Badlesmere. And so Leeds surrendered, and Edward strung them up.
Defeat in the Marches
Next up were Mortimer and Hereford. They had now retreated beyond the River Severn, looking to hold all the crossing's against the king. Still no help from Lancaster. But a lord called Gruffudd LLewyd in Wales remained loyal, and in their rear captured castles for the king. Mortimer threw in the towel, on the promise of the king's leniency - and then Edward chucked him in the Tower anyway. So Hereford fled north to join Lancaster.
Lancaster had found that the phrase 'King in the North' was just a phrase, not reality. He found it impossible to raise men against the king. He failed to realise that Edwrad would not just smile nicely, call him cousin and let him off. Lancaster's retainer Clifford eventually had to shake a sword in his face to get him to see sense - and Lancaster and Hereford fled north, heading for Dunstanburgh.
Sadly for them, Edward's Sherrif in Cumberland, Andrew Harclay held the bridge and ford at Boroughbridge. Lancaster attacked the ford, and Hereford the bridge. As Hereford laid about him, a spearman snuck underneath the bridge, and shoved his spear into Harclay's backside.
Lancaster called a truce to gather his energy overnight. In the morning he found that his army had melted away. Edward found him kneeling in a chapel.
The aftermath was a bloodbath. Lancaster was 'tried' - although he wasn't allowed to say anything in his defence - and beheaded. 6 more people were executed at the same time as Lancaster, and afterwards 117 families lost their lands.
The Declaration of Arbroath and a truce
In 1320 Bruce had issued the Declaration of Arbroath - a magnificent document well worth reading, and you can find it here or below. Edward had one more go at retrieving his reputation - taking north an army of over 20,000 men. He found a country empty of food, and was forced to retreat. Once again as he did so, the Scots re-appeared and fired the north of England. So at last Edward signed a truce - but no recognition yet of Bruce as a king.