On 16 September 1400, Owain was a proclaimed Prince Of Wales by his followers, following his disagreement with de Grey and his de facto treason against King Henry.
Gyndwr was popular as the Welsh nobleman with the greatest claim to the throne of Wales, and was able call on other disillusioned Welsh noblemens' support.
His first move was to sack the de Grey stronghold of Ruthin, before moving on to the towns of Denbigh, Rhuddlan, Flint, Hawarden and Holt. Oswestry followed, suffering massive damage at the hands of Glyndwr's troops.
With eight days, the force had moved as far as Welshpool and it, too, was sacked.
With such strong military success so quickly, other noblemen rallied to Glyndwr's standard. The Tudors of Anglesey, counsins of Owain and once close allies of Richard II, moved against English strongholds, employing guerilla tactics.
The military action happening across north and mid Wales forced Henry to bring his army back from a prospective invasion of Scotland. His initial skirmishes with the Welsh rebels were ignominious - harrassed by bad weather and the guerilla tactics of his aggressors. By 15 October he had retreated his forces back to Shrewsbury to lick his wounds.
Over the next few months, the revolt spread around Wales. Attacks rained in on English settlements, manors and castles, and in the south of the country, a group calling themselves Plant Owain (the Children of Owain), began mounting attacks around Brecon and Gwent.
In May 1401 the Tudor brothers seized Conwy castle when the garrison was at church. Owain himself overcame huge odds to rally 400 troops to victory over 1500 Pembrokeshire English and Flemish soldiers who charged their camp at the bottom of the Hyddgen valley. They killed 200 of the enemy took the rest prisoner.
King Henry was once more pushed into a punitive campaign, pressing for the Glyndwr-sympathetic abbey at Strata Florida in mid-Wales. He was attacked by the Plant Owain, suffering casualties from their guerilla tactics. His bad temper was such that after a marathon drinking session, he partially ruined the abbey and executed monks suspected of being Glyndwr loyalists.
Meanwhile, the Plant Owain continued their hit-and-run attacks on his supply lines, and on the way back to Hereford, the weather once more played its hand against Henry. Floods nearly washed the army away and the king himself almost died as his tent was blown over.
Complaints about the rising level of rebelliousness - including in the north west of England - gave rise to new laws which in the end served only to stoke the fires of rebellion, especially among those Welshmen hitherto remaining peaceable. There was now no pragmatic stance to take.
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