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I'm sorry, but there is no historical record of Wales gaining independence in the year 1400 CE. In fact, Wales was under English rule during this period, following the conquest of Wales by Edward I of England in the late 13th century (completed by 1283). The Statute of Rhuddlan in 1284 formally established English control over Wales, and the region remained under English authority throughout the 14th and 15th centuries.
However, the year 1400 is significant in Welsh history because it marks the beginning of the Welsh Revolt led by Owain Glyndŵr, a Welsh nobleman who declared himself Prince of Wales. Glyndŵr's rebellion was a major uprising against English rule, and for a brief period, he managed to establish a degree of Welsh autonomy. His revolt lasted from 1400 to about 1415, but ultimately, it was suppressed, and Wales remained under English control.
If you'd like, I can provide more details about Owain Glyndŵr's rebellion and its significance in Welsh history! Let me know.