What is the main conflict in the short story "Leiningen versus the Ants"?
This is a good question, because, while the obvious answer is that the conflict is between man and nature, an equally correct answer would be to say that the conflict is between man and his own fear.
Leiningen's Brazilian plantation is directly in the path of a swarm of billions upon billions of ants, which devour everything they come across. The local governor urges Leiningen to evacuate the area, but Leiningen is confident in his ability to outwit the ants. He firmly believes "that intelligence, directed aright, invariably makes man the master of his fate," and he therefore refuses to flee in the face of danger. He has taken many precautions against the ants, digging ditches around his property and filling one with water, the other with gasoline. The Brazilian governor calls the ants "elemental," and Leiningen intends to fight this "element" with the other elements of water and fire.
When the ants arrive, Leiningen's precautions are put to the test, and while they hold, he glories in the triumph of his intelligence over this challenge from Nature. His confidence inspires his workers with equal sang-froid, and as long as everyone remains calm, they are able to match each move the ants make with another move that forestalls the ants' advance. Once Leiningen's precautions begin to fail, however, "even he could not free himself from a qualm of malaise." He wonders if "this brain [had] for once taken on more than it could manage[.]" Still, he is determined to keep his chin up and solider on.
On the first day, there is a terrific battle to keep the ants from crossing the first ditch, which is filled with water. When some ants get across and attack one of Leiningen's native workers, the man screams in fear and pain, and Leiningen realizes "that another such casualty, yes, perhaps this alone, might plunge his men into confusion and destroy their morale," and that if that happens, they're all as good as dead. He manages to calm the injured man by instructing him how to remove the ants, and order is temporarily restored.
On the second day, the ants build little leaf-rafts to sail across the water-ditch. For a time, Leiningen is able to slow their advance, but then the ants get over the water and begin devouring the workers, who scream in agony.
When Leiningen heard this, he knew the plantation was doomed. He wasted no time bemoaning the inevitable. For as long as there was the slightest chance of success, he had stood his ground, and now any further resistance was both useless and dangerous.
Still he does not succumb to fear. He sounds the signal to retreat behind the second ditch, the one filled with gasoline, and his workers follow him:
And there, drifting in twos and threes, Leiningen's men reached him. Most of them were obviously trying to preserve an air of calm and indifference, belied, however, by their restless glances and knitted brows. One could see their belief in a favorable outcome of the struggle was already considerably shaken.
Fear is gaining the upper hand over the men's determination to resist the ants' onslaught, and their desire to believe in Leiningen's invincibility. Nevertheless, they decide to stay with Leiningen at the plantation, rather than take rafts to the other side of the river.
On the third day, the ants try to cross the gasoline-ditch, which Leiningen sets on fire. The ants are thrown into disarray by the flames, but only temporarily, and when the fire burns itself out, the ants mass again to reattempt the crossing. Leiningen "felt for the first time since the arrival of the ants that his confidence was deserting him."
[...] the ants were coming on again as if nothing had happened. And meanwhile Leiningen had made a discovery that chilled him to the bone-petrol was no longer flowing into the ditch.
Leiningen remembers that he has two final cisterns of gasoline in an outhouse, and in desperation, he and his men hook these up and fill the ditch again.
It was obvious, however, that this last resource meant only the postponement of defeat and death. A few of the peons fell on their knees and began to pray; others, shrieking insanely, fired their revolvers at the black, advancing masses, as if they felt their despair was pitiful enough to sway fate itself to mercy.
The men lose their nerve completely and begin to run through the fire blockade in an attempt to reach the river and swim to the other side. For every man who crosses the ditch, the ants swarm him and devour him, but the men are now so terrified that they are past reason. In desperation, Leiningen hits on one final way to get rid of the ants: dam the river and flood the entire plantation.
His confidence in this plan is largely bravado, but his men rally round him anyway and help him to accomplish it. To reach the dam, Leiningen must wade through the sea of ants, so he douses himself in gasoline to prevent their bites, and, with supreme bravery, begins the long walk to the dam-mechanism. The ants swarm him anyway, and bite him ferociously, but he is absolutely determined to dam the river, and dam it he does. When the water begins to flood the plantation, he staggers back towards his men but is almost overcome with pain from the ants' poisonous bites. It is only horror at the thought of being "picked clean" by their jaws that keeps him from sinking to the ground and never rising again.
And in the end, his men save him, the plantation is flooded, the ants are destroyed, and Leiningen's confidence in the face of total disaster has saved him and most of his workers from death. Courage has triumphed over fear.