The Krogan are a race of large reptilian-like creatures originating from the planet Tuchanka. Living on a planet known for its harsh and brutal climate, scarce resources, and dangerous predators, the Krogans became tough and hardened warriors while thriving under the harsh conditions of their world. Krogan society would become more and more advanced, also resulting in powerful and dangerous new weapons.
At the dawn of the Krogan nuclear age almost four thousand years ago, battles over the small pieces of land capable of sustaining life engulfed the planet. Tuchanka became a radioactive wasteland as these weapons were unleashed on each other. The Krogans would split up into seperate clans, all vying for supremacy over the other. Things would continue on this way until they were discovered nearly two thousand years later by the Salarians.
 Rise and Fall of the Krogans
Around two thousand years ago, were a primitive and warring race suffering through a nuclear holocaust of their own making. They were freed from their struggles by the Salarians, who supplied them with advanced pieces of technology and moving them to a planet that did not suffer from dangerous toxins and killer predators like the planet of Tuchanka.
The good Salarian deed did not come without a request of their own though from the Krogans. During this time, the Citadel forces were at war with the Rachni, a race of intelligent insect creatures. The Salarians hoped that the Krogan would join the Citadel in the battle since the Rachni were becoming a foe too much to handle on their own. Agreeing to fight, the Krogan were an unstoppable force as their numbers grew too fast for the Rachni to contend with, and within two generations, the Krogan had forced the Rachni back to their homeworld where they would be completely annihilated.
 Saviors of the Galaxy
Following their defeat of the Rachni, the Krogans were hailed as saviors of the galaxy. A statue was even erected inside the Citadel to honor their accomplishment. However, without the harsh conditions of Tuchanka to keep their numbers in check, the Krogan population grew at an acclerating rate. With their new planet overcrowded and running low on resources, the Krogan began exploring in space and taking other worlds by force. Oftentimes, they were planets already inhabited by races loyal to the Citadel. Tension reached its high-point after the Krogan forced their way onto an Asari based planet called Lusia. The Citadel Council ordered them to retreat off of the planet, but a Krogan Ambassador would have none of it. He challenged the Citadel to take the planet back themselves, and this would leave to an all out war.
The Krogan Rebellions would continue for around three centuries. The Krogan suffered through many casualties, but due to their incredible birth-rate, their numbers continued to grow. Realizing that defeat was imminent, the Council turned to the recently formed Turian Empire for assistance. The Turians unleashed the genophage on the Krogan home planets; A deadly bio-weapon developed by the Salarians. It's effects on the Krogan was everlasting; Infant deaths because the norm as only 1 in 1000 offspring would manage to survive, all but making the Krogan an extinct race.
 The Genophage
Without the ability to increase their numbers, the Krogan were forced to give up and accept the terms of surrender issued by the Turians. For their accomplishments in handling the difficult Krogan Rebellions, the Turians were offered a seat on the Citadel Council. To this day, the Krogan are still reeling from the effects of the genophage. Their numbers continue to dwindle, leaving them a suffering and dying race. Facing extinction, the Krogan have become rather individualistic, looking out for their self interest. They are often hired on as mercenaries as money is one of the few things to strive towards in their lives, while there are also Krogan who continue to despise every race associated with the Citadel races for bringing such a cruel fate upon their people.
Only the toughest and strongest of the Krogan would survive over time due to the brutal nature of the environment on their homeworld. Although not standing very tall, the Krogan contain quite a bit of muscle, giving them great durability during battle. Their large shoulder humps alow them to store fluids and nutrients, allowing the Krogans to go many days without need for food or water. Their thick skin allows them to become nearly impervious to most possible cuts and abrasions, while the Krogans are extremely resistant to most toxins and radiation, along with being able to survive in extreme heat or cold.
The Krogan also reproduce at a very high rate, being able to birth hundreds of offspring at a time. The youngest of the race oftentimes sport yellow and green markings on their back, a sign of their youth. Over time, these colors soon fade to give them more of a distinction. Along with being physically tough beings, there are also a number of Krogan who specialize in Biotics. Since the ability is rare with their race, the Krogan who do possess the strength needed for it are often extremely powerful.
Another useful distinction about the Krogan is the existance of multiple organs within their bodies. They almost serve as secondary systems in case a major organ suffers from disease or injury. This is one of the reasons why the Krogan are so difficult to harm or kill during combat since they offer such heavy resistance to damage. They also have a secondary nervous system that makes them almost impervious to paralysis. On top of that, the Krogan are a race that allow individuals to live for hundreds, and even thousands, of years.
 Military Doctrine
Traditional Krogan tactics were built around attacking directly with great numbers in an attempt to simply overpower and wear down the enemy. Supplied with cheap and rugged gear, troops were packed tightly together, making command and control very centralized. Soldiers in the field who saw a target contacted their commanders behind the lines to arrange fire support.
Since the genophage, the Krogans can no longer afford to charge in blindly and suffer mass casualties. This change of focus from mass-unit warfare to maximal efficiency has increased employment demand in the fields of security and 'muscle for hire'. Due to the unsavory reputation of the krogan, most of these jobs are on the far side of the law. The Krogan Battlemasters are one class of Krogan who are capable of taking on any ten soldiers of another species. They focus on developing clean, brute-force economy of motion that exploits their brutal strength to incapacitate enemies with a swift single blow of overwhelming power.
Battle Masters believe in being well-armed and equipped, preferably with a gun for each limb. They are callous and brutal, but methodical and disciplined. They use any means at their disposal to achieve their goals, no matter how reprehensible. Hostage-taking and genocide are acceptable means to ensure a quiet occupation with few krogan casualties. The krogan serving with Saren's forces appear to be returning to the old style of mass attritional combat. They also work in close cooperation with supporting Geth units, who fill in the roles occupied by combat drones in other armies.