Soon King Arthur had a large Kingdom and in the spirit of democracy he invited princes and knights from the surrounding areas to come and sit at his round table.
The greatest and strongest and boldest came to England to sit at King Arthur's court. They came from as far away as France, which wasn't a train's trip away at that time.
His strongest knight was by far Sir Lancelot, but others, such as Sir Gallahad, and Sir Gawain weren't too shabby either. They were held together by a common respect for King Arthur's judgment and progressive ideas like trials instead of direct execution. King Arthur was a heroic fighter, and a brilliant tactician, but of course something had to suffer. He paid little attention to his family.
This was a flaw in Arthur's character. From his childhood he was raised to think that his life's purpose was to be a great king. He was to care for his subjects and be wise, and noble. He neglected Guenevere and soon she was having an affair with Lancelot.
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Soon everyone knew what was going on. Lacelot probably bragged about it. Arthur even knew but didn't want to know about it. It was proven to the King that it was true, and in his rage Arthur decided to have Guenevere burned at the stake. Which isn't keeping with his normal Great, benevolent king image. So in Le Morte D'arthur Lancelot rides in and saves her. Killing two of Gawain's brothers who vows revenge. So a civil war occurs which in the end rips the kingdom apart.