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The Krogans reach the capabilities of nuclear warfare.
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And thus, they attack each other and start a nuclear winter.
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At this time, other alien races have found the rachni, a hostile insect race. They find that they can’t fight back, so they recruit, or rather, uplift the Krogans to fight for them. The Krogans won, were admired by all, even granted a seat of Council. Until, Krogans began overpopulating.
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After the long fought rebellion, the Krogans surrender once the Salarians and Turians disperse the genophage, a genetically modified disease that effectively renders the Krogan race, sterile.
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The Krogan have dwindled, heading steadily for extinction. Of course, they've been looking for a cure all this time, but alas, none has been found. Instead, due to their aggressive culture and lack of hope, they've killed themselves in wars.
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And because of overpopulation, the Krogans began warring amongst themselves for land, and eventually into the stars. But, the big mistake they made was conquer a planet. This made the other races retaliate, instigating the rebellion.
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Mordin reveals that the Krogan could(although a miniscule possibility) populate themselves away from extinction, but the Krogan attitude won't allow for it.
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In which the genophage is either cured or left alone. It depends on the player's choice mostly.