Read this line from the war of the worlds: the immediate pressure of necessity has brightened their intellects, enlarged their powers, and hardened their hearts… what does the phrase "hardened their hearts" imply?

Implies they are less sensible or suffer less with what their hearts feel.

Explanation:

The phrase “hardened their hearts” in this excerpt of The War of the Worlds is like a metaphor, because the heart itself cannot be hardened, it means they were stronger or were made harder with their feelings and emotions due to the pressure of necessity, “hardened their hearts” implies they are less sensible or suffer less with what their hearts feel.