The turians, 2160-2175
For 1300 years, the Turian Hierarchy existed as the uncontested military giant of the galaxy. All the other races of the galaxy combined, including the other Council powers, the asari and the salarians, just barely matched the turians in military power. None of them had the marital tradition that the turians possessed save the krogan, who where but a battered and beaten ruin of their former strength. Simply put, in the entire galaxy their existed but one Superpower, and that was the turian military.
Since about halfway through the Krogan Rebellions, the turians had experienced nothing but a string of uncontested triumphs. A few small skirmishes were lost, and on occasion the turians would lose something approaching a major battle. However, these were always followed by a campaign that utterly annihilated their foes and placed the turians as the total victors of the conflict. None of the losses were to major powers, but instead to short coalition of pirates and warlords that never actually constituted a real threat to turian dominance.
In 2158, the galaxy was turned on its head. For the first time in living memory, even that of the millenia old asari matriarchs, the turian Hierarchy suffered a true defeat. An entire battlefleet with no fewer than three dreadnoughts was lost. The turians had not lost a single dreadnought since 50 years before the end of the krogan rebellions, yet now they had lost a total of four in the space of a few short months. In the end, the human and quarian alliance was forced to sue for peace after a major turian victory at Pheiros, but the damage was done. The turians were no longer invincible.
In many species, such a bitter defeat would have resulted in extensive purges of the military leadership and large scale demoralizing in the rank and file. With the turians, the complete opposite happened. Instead of hating the newly formed League, many turians respected them. It wasn't a respect born of love, but of a grudging admiration that they had finally found a worthy foe. Turian military doctrine underwent extensive changes in a very short time, and new fleet design based on new technology and concepts was pushed into action. The use of carriers, combined with stealthed frigates to attack the enemy's flanks and capital ships was created. New weapons technology was introduced, and in the next 20 years after the League/Turian Conflict, the turian military reasserted itself as the premiere military power.
Many turians eagerly awaited what was widely considered to be the inevitable renewal of hostilities Here was a chance to prove the turian way was the right way, to regain their lost honor. When the krogan joined the League, a good percentage of the turian military was privately excited. They would have a chance to prove they were heroes in the same vein as their forefathers. The Hierarchy used its superior economic power and manufacturing base to increase it's fleet size and strength far past anything the galaxy had seen in 50,000 years.
What few realized at the time was that while the Heirarchy would prove itself heroes once more, and that the turian military would be both sword and shield to a galaxy beset by foes, they would not do so against the humans. Nor would they do so alone.