What triggers the battle between King Arthur and Sir Mordred?

On a literal level, when Arthur leaves the court to fight Lancelot over his presumed affair with Guenivere, he places Mordred on the British throne. Mordred begins to form allegiances with false promises, and the people resist Arthur's return. At this level, the war is a power grab, and Arthur fights the civil war to regain his throne.

On a more symbolic level, we sense that Mordred stands for an inner force, caused by human failing (Arthur's affair with Morgause). Camelot was superior to all other nations and could not be defeated in battle. Its weakness is internal, like a classical "hamartia" (fatal flaw). Mordred represents the consequence of Arthur's and Camelot's weakness.

Without Launcelot or Gawain, Arthur returns to claim his throne profoundly lacking in his most trusted companions. He was warned to not fight until Launcelot could rejoin Arthur, but in the fatal truce ceremony, a snake accidentally crosses a knight's foot. Instinctively, he draws his sword to kill the snake, but this is misread as a sign of aggression and a betrayal of the peace terms. War breaks out at this point.

This seemingly meaningless accident leads to the brutal battle that leaves only a handful of Camelot's knights standing. In this way, we can read the war as a series of accidents and betrayals dating back to Arthur's affair, Launcelot and Guenivere's affair, Gawain's wounds from fighting Launcelot, Launcelot's delay in coming to Arthur's aid, Mordred's political contrivances rather than honor, and a snake.

What triggers the battle between King Arthur and Sir Mordred?

Mordred is Arthur's illegitimate son. And Merlin prophesies that, if the boy is allowed to grow up, he will one day destroy both Arthur and his kingdom. That being the case, the wily old wizard urges Arthur to round up all the babies born on May Day—Mordred's birthday—and have them put out to sea. Yet the infant Mordred somehow manages to escape the ensuing massacre and lives to fight another day. When he grows to manhood he becomes his father's most implacable foe, hell-bent on destroying Arthur and taking his kingdom for himself.

As well as being a traitor, Mordred routinely flouts the code of chivalry to which knights are expected to adhere. For instance, he kills Sir Lamorak by stabbing him in the back, an act considered deeply dishonorable for any knight of the realm. Mordred is so power-crazed, so obsessed with grabbing power, that he unilaterally declares himself king and tries to marry the queen. And all this before he actually kills Arthur at the Battle of Salisbury Plain.

Describe the battle between Arthur and Mordred in "Le Morte d'Arthur".

In Malory’s Le Morte d'Arthur, as king Arthur comes back to Britain, he finds out that Mordred, his son and vicegerent, has usurped his throne. The king wants to bring the traitor to justice, but he sees a dream in which Sir Gawaine advises him against going into battle with Mordred. Otherwise, he will be killed. This reminds Arthur of Merlin’s prophecy “that there should be a great battle beside Salisbury, and Mordred his own son should be against him.” (Volume 1, Chapter 11) So the king appoints Sir Lucan the Butler and his brother Sir Bedivere with two bishops as envoys to Mordred to negotiate a peach treaty for one month.

They come to Mordred, who has amassed a huge army of one hundred thousand men, and persuade him into making peace with the king. Mordred agrees “for to have Cornwall and Kent, by Arthur’s days: after, all England, after the days of King Arthur.” (Volume 2, Chapter 3) The decision is made that Arthur and Mordred, each accompanied by fourteen men, will meet between the two armies. However, the king does not trust the traitor. So Malory comments:

And when Arthur should depart, he warned all his host that an they see any sword drawn: Look ye come on fiercely, and slay that traitor, Sir Mordred, for I in no wise trust him (Volume 2, Chapter 4).

Mordred addresses his army with a similar warning, because he fears that his father is going to take revenge on him for the treachery.

They meet, negotiate, and drink wine as a sign of armistice, if not reconciliation. But fate deems otherwise. An adder comes out of a heath bush and stings a knight on the foot. The knight sees the adder and draws his sword to kill it without thinking about the consequences of this involuntary action. This triggers the renewed hostility:

And when the host on both parties saw that sword drawn, then they blew beams, trumpets, and horns, and shouted grimly. And so both hosts dressed them together (Volume 2, Chapter 4).

Then comes one of the most beautiful passages in the whole of Malory’s romance, summarizing the climax of Arthur’s glorious feats and giving the story biblical proportions:

And never was there seen a more dolefuller battle in no Christian land; for there was but rushing and riding, foining and striking, and many a grim word was there spoken either to other, and many a deadly stroke. But ever King Arthur rode throughout the battle of Sir Mordred many times, and did full nobly as a noble king should, and at all times he fainted never; and Sir Mordred that day put him in devoir, and in great peril. And thus they fought all the long day, and never stinted till the noble knights were laid to the cold earth; and ever they fought still till it was near night, and by that time was there an hundred thousand laid dead upon the down (Volume 2, Chapter 4).

As the battle ends, Arthur notices that all of his knights have fallen except Lucan the Butler and his brother, Sir Bedivere. He bewails his companions’ deaths. But he wants to deal with Mordred who stands leaning upon his sword among the dead bodies. He asks Lucan to bring him his spear. But Lucan dissuades the king from taking revenge on Mordred, reminding him of his recent dream that resonates with the old prophecy.

Yet Lucan’s words fall on deaf ears as the king, moved by vengeance, rushes towards his foe. Mordred, filled with hatred and rage, dashes forward, armed with his sword. Arthur pierces Morderd with his spear, but Mordred strikes him on the helmet with the sword, which mortally wounds the king. Mordred falls dead at once, but Arthur is taken to a small chapel near the sea.

Lucan dies of wounds, and Bedivere brings Arthur, at his behest, to the seashore and places him into the barge which is to take him to the vale of Avilion (Avalon) where his wound is to be healed.