Many quotes and lines from The War of the Worlds have gone on to act as quintessential science-fiction tropes in literature and film. For example, the very first lines:
No one would have believed in the last years of the nineteenth century that this world was being watched keenly and closely by intelligences greater than man's and yet as mortal as his own...
This idea became the seed for thousands of alien invasion stories that followed the novel's success. The idea that there are other intelligences in the universe was already in the public consciousness, but this story served to codify the intelligence of alien as specifically non-human, and possibly impossible to understand.
Another important quote comes near the end, as the Narrator realizes that the Martians did not properly protect themselves from Earthly disease:
...the Martians--dead!--slain by the putrefactive and disease bacteria against which their systems were unprepared... slain, after all man's devices had failed, by the humblest things that God, in his wisdom, has put upon this earth.
(Wells, The War of the Worlds, eNotes eText)
This shows an underlying theme of the novel, that Humankind cannot necessarily defend themselves. Instead, it is the process of evolution, of the Human ability to adapt and survive, that the Martians do not have; it is likely that the Martians have not evolved in millions of years, only progressing mentally, and so were entirely unable to defend themselves against simply disease bacteria.
No one would have believed in the last years of the nineteenth century that this world was being watched keenly and closely by intelligences greater than man's and yet as mortal as his own; that as men busied themselves about their various concerns they were scrutinised and studied, perhaps almost as narrowly as a man with a microscope might scrutinise the transient creatures that swarm and multiply in a drop of water.
This is the opening line and it set a precedent for future science fiction works.
And before we judge of them too harshly we must remember what ruthless and utter destruction our own species has wrought, not only upon animals, such as the vanished bison and the dodo, but upon its inferior races. The Tasmanians, in spite of their human likeness, were entirely swept out of existence in a war of extermination waged by European immigrants, in the space of fifty years.
This quote shows the implications of exterminating a race. It compares European imperialism to the Martian attack.