The years 1404 and 1405 were perhaps the high point of Owain's rising. During 1404 he had control of almost all Wales, and his enemies were reduced to the occasional foray from one of their remaining garrisons. He was able to conduct diplomacy with France and Scotland, and even to hold parliaments. He removed the Welsh Church from the dominion of Rome and adhered to the rival pontiff at Avignon. He even received French military assistance.
How useful this was is in some question. One suspects the French knights had a bit of a culture shock in Wales. According to some accounts Owain advanced, with French aid, to Woodbury Hill, some eight miles from Worcester, where he was faced by Henry IV and an English army. Neither side feeling strong enough to attack the other, the Franco-Welsh army backed down and retreated.
R R Davies questioned whether this actually happened, and the fact that there is doubt demonstrates the difficulty of adequately describing the Glyn Dwr years, and maybe also the power of myth.
Anyway, in 1405 the famous Tripartite Indenture was signed between Owain, Edmund Mortimer and Northumberland to divide the kingdom between them. This was probably a reaction to the failure to get Mortimer's nephew, March, into their hands, but was a classic case of counting chickens that hadn't been hatched.
Henry IV was preparing a further massive invasion of Wales, but in the event had to take his troops north to deal with Scrope's rising - as usual the rebellious elements failed to get their act together and the King was able to deal with them in detail. Though he had not long since spared his York cousins from execution, Henry was obviously in a ratty mood, as he had Scrope (the Archbishop of York!) and Thomas Mowbray, the young Earl Marshal, summarily beheaded.
Henry fell ill almost at once (divine intervention perhaps?) and was never quite the same man again. His visits to Wales were over for good, but his cause there began to prosper.
In the spring of 1405 Glyn Dwr's forces in the south of Wales suffered heavy defeats in successive battles at Grosmont and Pwll Melyn at the hands of Richard Beauchamp Earl of Warwick and the Talbot brothers. Owain's brother was among the dead and his eldest son, Gruffudd, was captured and apparently spent the rest of his life in the Tower. His brother-in-law, John Hamner, was also captured but got away with a fine of 500 marks, a huge amount for someone of his relatively modest standing.
The rising was far from over, but this was the beginning of the end. In August commissioners were sent to the men of Usk and Caerleon to discuss terms of surrender. Meanwhile, in the north, Gwilym ap Gruffudd ap Gwilym, the most powerful man in Flintshire, surrendered himself to be imprisoned at Chester, along with his brothers and four supporters. Anglesey, the bread basket of Wales, was raided by royal naval forces based in Ireland. The island was to be forced into submission during the following year.