The War began after Daleks began attacking and destroying Human colony ships, transports and probes, causing a massive outcry to the League. However, the League did not want to attempt to sanction the Daleks for fear of retaliation, so it did noting. Humankind instead took matters into its own hands, summoning a fleet of thousands of warships to meet the Daleks in open combat. However, the Daleks had other plans. They started attacking other races too, races too weak to fight back, and soon the entire Galaxy was in outcry as planet after planet fell.
This process continued for several weeks, of the Humans winning isolated victories but the Daleks sweeping in and destroying everything. Several planets were wiped from the face of the Galaxy, utterly destroyed by the Daleks constant bombardments. Such a level of destruction at the hands of the Daleks had never been seen before, and would not be seen again until the stories from the Time War that soon followed came to light in the Galaxy.
In their quest for galactic destruction, the Daleks wiped entire systems clean of life, spreading from Galactic arm to Galactic arm like a virus, eradicating everything in their path that was not strong enough to resist, and causing massive socio-economical damage to the League as a whole. As the Dalek war machine closed its fist around large and populous colony worlds like Sau'dos and Eron Major, the results were disastrous - a trillion deaths were reported in one day as the Daleks simultaneously conquered 14 solar systems, and the Daleks spread their forces thinly across League space to maintain their ambitious gambit to cleanse the Galaxy of life.However, as a result of this aggressive strategy, the Humans were eventually able to break a hole in the Dalek's defences, exposing their only true weakness - their homeworld. Filled with Dalek secrets, Skaro was a prime target for the Humans, and the Daleks knew it. After intercepting heavily incrypted transmissions hinting of a combined Human attack on Skaro, the Daleks positioned thousands of battleships around their home planet to intercept the Human combined fleet and wipe their foe from the universe once and for all. However, it transpired that the entire operation was a gigantic deception. The Humans were in fact never even thinking about attacking Skaro, they were more interested in destroying the Dalek Crucible, which was a massive production facility for Daleks and yet another prime target - now left relatively undefended after the Daleks had reassigned their fleet to Skaro.
At this point, the League army finally decided to step in. Ex-Covenant warships and Reaper fleets amassed around the Dalek Crucible, and the Human fleet hung back to watch the fireworks. However, the fireworks never came. The Daleks instead activated a weapon hidden for millenia - the Reality Bomb. Used on a low power setting, it eradicated the crews of all the ships sent against them, as Elite, Reaper and Turians alike were eradicated in a single sweeping move, similar to the tactics employed millenia earlier by the legendary Forerunner AI known as Offensive Bias .
Shocked, the Human fleet had to go against the Daleks alone. The Reality Bomb had offlined the Crucibles major systems, leaving it vulnerable, so the Daleks swarmed out of their hive to attack the Humans. After a long and bitter battle involving some ingenious tactics, for example using empty Ex-Covenant ships as makeshift bombs, the Human fleet eventually overcame the Dalek charge and forced them to retreat back into the Crucible, which then utilized the last of its power to teleport away to parts unknown.
After saving the League from the power of the Daleks, the Humans were given special rights and were ascended into the higher circle of respected leader races of the League to fill the void of higher races that had been exterminated. After the disappearance of the Crucbile, the Dalek race itself appeared to disappear - their outposts were abandoned, and sieges on larger colony worlds abruptly ended. The Second Galactic War was over, but the conflict was not. The Time War soon followed, with the Daleks amassing for war against the Time Lords, a war which left both races on the verge of extinction and the Daleks weak and vulnerable.
The war not only proved that the Daleks could be beaten, but it also opened doors for the negotiations with the Daleks that occured after the Time War. This soon led to a reformation of the Dalek Hierarchy and a return to a more peaceful Dalek race, which then rejoined the League once again as a member of its military.