Is there a similarity between the breaking of Narsil and the loss of the One Ring by Sauron?

Question

I wonder whether there is any suggestion in Tolkien's books that the breaking of Narsil led to the weakening of the strength of the men of the West and thus to the corruption of Isildur and the fall of the Kings of Arnor. Is Narsil said to hold the same magical powers within its forging the same way the One Ring holds within the malice and power of Sauron? Is therefore Anduril for the Men of the West what the One Ring is for Sauron?

Answer 1

No, not at all. While you could perhaps make a lit-crit argument that the loss of the Ring and the breaking of Narsil parallel each other in some symbolic way, there's no indication in the text itself that they are similar in any mechanical way.

Is Narsil magical?

There is no evidence of Narsil/Andúril had any magical power, beyond what you would find in a normal sword of Elvish make:

[T]he sword of Elendil filled Orcs and Men with fear, for it shone with the light of the sun and of the moon, and it was named Narsil.

^{The Silmarillion V OF the Rings of Power and the Third Age}

Aragorn threw back his cloak. The elven-sheath glittered as he grasped it, and the bright blade of Andúril shone like a sudden flame as he swept it out.

^{Two Towers Book III Chapter 2: "The Riders of Rohan"}

Andúril rose and fell, gleaming with white fire.

^{Two Towers Book III Chapter 7: "Helm's Deep"}

What limited power it does have is mostly due to its reputation as the sword that defeated Sauron, not any especially magical power.

Is the breaking of Narsil related to the diminishing of Men?

Not at all. The diminishment of Men began long before, when the Men of Númenor turned against Eru Ilúvatar:

In those days the Shadow grew deeper upon Númenor; and the lives of the Kings of the House of Elros waned because of their rebellion

^{The Silmarillion IV Akallabêth}

Later on, it was worsened by the Gondorians intermingling with non-Númenóreans:

Yet at the last, in the wearing of the swift years of Middle-earth, Gondor waned, and the line of Meneldil son of Anárion failed. For the blood of the Númenóreans became much mingled with that of other men, and their power and wisdom was diminished, and their life-span was shortened, and the watch upon Mordor slumbered.

^{The Silmarillion V OF the Rings of Power and the Third Age}

Nothing to do with Narsil.