What is the theme of 'The Convergence of the Twain'?

To answer this question you need to understand Hardy's own
philosophy of life and how he viewed events and tragedies such as the sinking of the
Titanic, which is the concern of this poem. What is notable about this poem is that
Hardy does not dwell on the loss of life. What concerns him is the way in which the
sinking of the Titanic, the ship that was supposedly unsinkable and was the triumph of
human achievement, is a demonstration of how the gods or fate chooses to attack humans
that are arrogant in their own capabilities and powers. Note the reference to "human
vanity" in the first stanza:

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In a solitude of the
sea

Deep from human
vanity,

And the Pride of Life that planned here, stilly
couches she.

There is an
intense irony in the scene of the Titanic "couching" quietly, fathoms from the pride and
hubris that led, Hardy argued, to its sinking.

This poem
therefore establishes the futility of man's attempts and efforts to gain control over an
indifferent universe that demonstrates how powerless mankind really is. The Titanic was
the height of technological sophistication at its time, and thus its sinking confirms
Hardy's philosophy of an "Immanent Will" that "stirs and urges everything" against
mankind. Humans are shown to be ultimately powerless and defenceless against the might
of nature and the divine, and this is powerfully expressed, according to Hardy, in the
sinking of the unsinkable ship.