There are a number of historical sources that mention a figure named Arthur. The earliest of these is the Historia Brittonum, written by the Welsh monk Nennius in the 9th century. Nennius mentions Arthur as a great warrior who fought against the Saxons in the 6th century. Other early sources that mention Arthur include the Annales Cambriae and the Gododdin.
However, there is no evidence that any of these sources are based on actual historical events. It is possible that the stories of King Arthur were originally based on a real person, but over time they became increasingly exaggerated and embellished. It is also possible that the character of King Arthur was entirely invented by medieval writers.
The debate over whether there was a real King Arthur is likely to continue for many years to come. There is simply not enough evidence to say for sure whether the character is based on a historical figure or not.