The army of the dead always intrigued me, and they have a really interesting story. Also known as the oath breakers, the Dead Men of Dunharrow were once asked to came to the help of Isildur when he went to fight against Sauron. However they did not come to his aid when called upon despite the oath of allegiance they pledged to him. Because they broke their promise they were refused the opportunity to have peace or rest until they fulfilled their oath. They were miserable, and had they kept their oath, peace and rest would have been the outcome.
Some of us, like the dead men, have made oaths or covenants through baptism. We promise to join the Savior's ranks to be a light to the world and an example to those who do not know him. The promised reward is the constant companionship of his Spirit which blesses us with peace and joy. However, if we do not keep those covenants then we will have to live knowing that we let the Savior down.
But none of us are perfect, and like the story of the army of the dead, there is always the opportunity to repent and be forgiven. When we mess up, the King of Kings always gives us another chance to fulfill our oath-to keep our covenants; and even though falling short may have kept us from that peace, he is always waiting to extend that to us as long as we ask for forgiveness.
Alma 34:31 in the book of Mormon states:
"Yea, I would that ye would come forth and harden not your hearts any longer; for behold, now is the time and the day of your salvation; and therefore, if ye will repent and harden not your hearts, immediately shall the great plan of redemption be brought about unto you."
So make and keep covenants! They bring peace and happiness, and even if we fall short God is always waiting to extend his arm of mercy towards us.