Battle of Phall
The Battle of Phall is one of the biggest space battles of the entire Horus Heresy and, given that the war took place over an entire galaxy that's saying something. It and the lead-up was detailed in the Horus Heresy novella, The Crimson Fist.
The Contenders
On team Emperor we have the Retribution fleet, a full third of the VIIth legion fleet; initially under command of Captain Yonnad, who was dead, and command fell to Alexis Polux. The Fists were known to have have the largest fleet of all the Legions.
On team Horus we have Perturabo, Primarch of the Iron Warriors and his entire legion's war fleet, which consisted of some of the biggest and most well armoured starships that had ever been built. And of course, a massive grudge against the Imperial Fists that he could finally act on.
The coming of the storm
Following the flight of the Eisenstein, Lord Rogal Dorn heard about Horus' plan to crash the Imperium with no survivors and dispatched a third of his Legion and a bigger part of his war fleet to take Horus to task. This Retribution fleet was sent ahead of the Raven Guard, Iron Hands and Salamanders who followed shortly after. Unfortunately, the Retribution Fleet was marooned in the Phall system since the Chaos Gods thought that odds of 7v4 weren't stacked enough in their favour. Yonnad unfortunately died during the warpstorms and command was thrust on one Alexis Polux. Polux was suspicious of the calm around the Phall system when everywhere else was fucked as far as the third eye could see and began rigorously drilling and training the men under his command, which turned out to be a very good thing when the fleet of the traitor IVth turn up.
The Battle of Phall
Maintaining a defensive position near a planet, the Imperial Fist ship groups began looping around a point in space to form a sphere of guns. Polux was right about the suspicious nature of the system, and sure enough the main Iron Warriors fleet showed up to kill them. Unfortunately for the Iron Warriors, Perturabo thought that Sigismund would be in command of the Retribution Fleet. Sigismund was expected to do two things: land on the planet and build a fortress and charge at the Iron Warriors fleet guns blazing with the remaining ships. However, Sigismund decided to listen to a witch instead of his Primarch and was disowned and stripped of command of the fleet. Polux's fleet maneuverability and powers of narrative favoritism evened the odds and gave the Iron Warriors a much harder time than they were expecting.
In the middle of the battle, Pollux got a call from Terra with order to withdraw ASAP. Given the choice of fighting it out to the bitter end or following orders and exposing their collective back to the Iron Warriors, Polux decided Papa Rogal needed as much of his fleet home as he could manage. Polux gambled and ordered his ships to break away and run for it. In the end, the gamble payed off in the sense that it made a bad situation from becoming a disaster; the Imperial Fists paid for showing their backs, but the butcher's bill was lighter than standing and continuing the fight. Polux's fleet retreated, leaving the battle a strategic draw.
And because GW doesn't like traitor marines to have too many nice things, Pert's own postmortem showed that he would have lost if the battle continued; despite the fact he had the larger fleet, the biggest and mostest guns, and more tactical assets. The Imperial Fists were better equipped (being in the same system as Mars after all and having shit like vortex missiles) and could punch above their weight, and Polux's stratagem enabled the Fists to adapt quickly to the Iron Warriors attack. (Perturabo had the Iron Warriors charge into the Imperial Fists in a jagged cone formation, which was apparently less effective at countering the Imperial Fists revolving sphere formation. Also, the slower Battleships drew fire and the faster ships pulled off hit and run attacks.)
No wonder Pert is pissed off all the time.
Also, during all of this, an Imperial Fists boarding party of Terminators boarded the Iron Warriors's flagship, hunting for Perturabo and some surviving just long enough to reach his quarters only to be told to bend over. One of them survived, a plot hook which was wasted in Angel Exterminatus, as the character was a detail of a subplot and overshadowed by another Fist introduced at the start of that book.
And some of the Imperial Fist ships got back to Terra... Oh wait that wasn't the Astronomicon. Welcome to the Imperium Secundus.