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To start, imperialism is defined as...
...the domination of one state over a number of others.
In Joseph Conrad's Heart of Darkness, the author gives vivid imagery describing conditions when Charlie Marlow arrives at the lower station in the Congo.
Imperialism is noted first in studying the reason Marlow is originally hired to travel into the jungle:
...the company had received news that one of their captains had been killed in a scuffle with the natives...it was only months... afterward, when I made the attempt to recover what was left of the body, that I heard the original quarrel arose from a misunderstanding about some hens.
The captain, a "Dane" acting out of character, had taken his anger out on a village chief who he started beating with a stick—over a misunderstanding involving chickens. One of the "thunderstruck" natives that was watching—the chief's son—attacked the captain and killed him.
When Marlow finally arrived to find the remains where the captain had dropped—grass growing through his bones—he was stunned to find the village empty.
...the village was deserted, the huts gaped black, rotting, all askew within the fallen enclosures...The people had vanished. Mad terror had scattered them, men, women, and children, through the bush and they had never returned.
Marlow describes the dead captain:
The supernatural being had not been touched after he fell.
This shows how the natives perceived the white man—a mystery who brought violence with it—and fear that more would come. And they had.
Another vivid description to exemplify the morally reprehensible results of imperialism is found in Marlow's description of the enslaved natives who are being forced to work for the company.
Six black men advanced in a file, toiling up the path. They walked erect and slow...and the clink kept time with their footsteps. Black rags were wound round their loins...I could see every rib, the joints of their limbs were like knots in a robe; each had an iron collar on his neck, and all were connected together with a chain...All their meager breasts panted together, the violently dilated nostrils quivered, the eyes stared stonily uphill. They passed me within six inches, without a glance, with that complete, death-like indifference.
The men are followed by a guard with a rifle, who shows Marlow respect because he is white—Marlow notes that there was no way from a distance the man could have told specifically who he was: the respect was simply because of the color of Marlow's skin, and as they passed, the guard "...seemed to take me into partnership in his exalted trust." Marlow is perceived a part of the powerful form and force of imperialism, which has subjugated these innocent natives, turning them from men into "criminals", into slaves, though he is no more than a witness.
Moving on, Marlow finds a group of natives who are not working, but neither are they being afforded a rest or an opportunity to eat:
Black shapes crouched, lay, sat between the trees leaning against the trunks, clinging to the earth, half coming out, half effaced within the dim light, in all the attitudes of pain, abandonment, and despair... this was the place where some of the helpers had withdrawn to die.
While the company makes millions on the ivory trade, the natives are crushed, in the name of imperialism.