He studied law in London and then served with the forces of Henry Bolingbroke, an opponent of King Richard II, later to seize the throne and become King Henry IV.
On his return to Wales, he found that England's oppressive rule had paralysed the Welsh economy and aroused popular resentment.
In September 1400, a year after Bolingbroke usurped the throne, Glendower's feud with a neighbour, Reynold, Lord Grey of Ruthin, touched off an insurrection in north Wales. The insurgency quickly became a national struggle for Welsh independence.
Glendower formed strategic alliances with Henry's most powerful opponents. In 1402 he captured Sir Edmund de Mortimer, whose nephew the 5th earl of March had a claim to the English throne, and secured his support under. He then allied himself with the Percy family (Sir Henry Percy; Henry Percy, 1st Earl of Northumberland; and Thomas Percy, Earl of Worcester). The defeat of the Percys at the [Battle of Shrewsbury]? in 1403 (in which Owen did not take part) was a temporary setback for the Welsh leader
By 1404? he had gained control of most of Wales. Styling himself prince of Wales, he established an independent Welsh Parliament and began to formulate his own foreign and ecclesiastical policies.
In 1405? the tide of battle turned decisively against him. The failure of an expedition from France on his behalf (1405–6) weakened him. The recapture by the English of Aberystwyth (1408) and Harlech (1409) under Prince Henry, later [Henry V]?, left him powerless. His allies in England were crushed. He was, however, active in guerrilla fighting as late as 1412.
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