Peter Jackson's film adaptations of the Lord of the Rings trilogy focus a lot on the importance of kings, and their love for their kingdoms and their peoples. The film also shows the reverse of this, and how these once-mighty dominions can full to ruin and despair when left in the hands of inferior rulers. This can be seen predominantly in the portrayals of Denethor, the Steward of Gondor, and Theoden, the King of Rohan.
Both Theoden and Denethor teach the audience something about the strength and the will of Mankind. They represent the same pathway, and they have the same fears for the future of their kingdoms, but they make very different choices, which ultimately lead to very different outcomes for their characters. In order to understand how and why their paths diverge, it's important to start by looking at the similarities in how they begin. First and foremost, is that when these characters are introduced in TheTwo Towers, both are being corrupted by influences outside of themselves, which has left them weak and aged before their time, and their kingdoms open to attack and ransacking.
For Theoden, the influence comes from Grima Wormtongue, who is infecting the mind of the king with warmongering and ill-counsel. In the chapter ‘The King of the Golden Hall’, Gandalf says of the man "For ever Wormtongue’s whispering was in your ears, poisoning your heart, chilling your thought, weakening your limbs, while others watched and could do nothing, for your will was in his keeping."
By the time that Gandalf and the other members who travel with him arrive in Rohan, Theoden is already stooped, greying, and beyond all reasoning with. Similar can be said of Denethor, but his suffering and aging come from a different exterior source. For Denethor, it is the use of the Palantir, one of the Seven Seeing Stones left in Middle Earth (that aided in the corruption of Saruman) which sits in a tower at Minas Tirith, that absorbs his strength and leaves his mind tainted. He, like Theoden, is seen to be growing old before he should be, as he has used the last of his power in trying to defy the will of the Evil Lord Sauron when he looks into the palantir to see the growing threat at the edge of his borders.
Both characters are also heavy of heart for another reason: The death of their eldest sons and heirs. Theoden mourns the loss of his child Theodred, laying him to rest in the hills with his forefathers. He has previously sent his remaining male heir, Eomer, the leader of the riders of Rohan, away at the behest of Wormtongue, but is able to call him back and re-affirm his loyalty. Denethor is broken by the loss of his eldest Boromir, who died protecting Merry and Pippin from the Orcs who attack the fellowship. He is so bereft that he cannot see clearly through the grief and sends his remaining male heir Faramir almost to his death too.
It is at this point, however, that their paths diverge, and the choices they make lead them to their respective triumphs and downfalls. When faced with the possibility of impending doom, in which the Dark Lord will take over Middle Earth and destroy all that is good and beautiful in the world, Theoden chooses the path of hope. He chooses to ride out with his warriors and fight, showing that the will of Men cannot be broken, and that there is still nobility and leadership to be found within the kings of old. He warns that blood will be spilled, and that lives will be lost, but ends his famous speech with the words ‘the sun rises’, meaning that there will be a future full of light and hope for those who survive. In the subsequent battle, Kind Theoden does die, after watching his niece Eowyn sacrifice everything to try and save him, but his death is an honorable one, and he has earned the right to join his forefathers with a place of heroism, "in whose mighty company he shall not now feel ashamed."
Denethor, on the other hand, when confronted with the same choice, chooses the path of fear and despair. Although there are some changes in Denethors character between the books and the movies, one thing that remains consistent is his choice to go to his death. He knows somewhere deep down that Faramir is alive, but he refuses to acknowledge it because he has already given up, and wants nothing more than to escape the trials and the darkness to come by going to his fiery grave, and taking Faramir with him. He sets himself upon the pyre with his son, telling Gandalf "You may triumph in the field of battle for a day, but against the power that has risen in the east, there is no triumph." This demonstrates what happens when the strength of Men fails, and the leader succumbs to darkness and cowardice. Gandalf and Pippin are able to rescue Faramir at the last moment, and Denethor dies in the dramatic cinematized scene where he falls from the parapet of Minas Tirith, thus ending his rule and making way for the true King of Gondor, Aragorn.
Despite their very different endings, both Theoden and Denethor are richly portrayed characters with nobility and pride, and Lord of the Rings fans everywhere hope that they were both able to find some form of peace within their final rest.
Source: The Lord of the Rings book by J.R.R.Tolkien