As a whole the Horus Heresy can be boiled down to a few key battles. While the novels expand the campaign into a sprawling network of skirmishes and conflicts, the core of the actual war can be put down to the two Isstvan battles, Prospero, Calth, Signus Prime, and then Terra. The outcome of each was critical to the overall victory of one side or the other, but the point where it all could have been ended has always remained the same - Isstvan V.
Better known as the Drop Site Massacre, the campaign was the Emperor's response to learning of Horus' betrayal. Now forewarned that his legion and four others had turned, botching their initial strike by being dragged into a bloody and costly conflict with loyalist elements of their own legions, he sought to end it with a hammer blow.
After a substantial number of Imperial Fists were sent to try and assist the loyalists at Isstvan III in any way they could, forces from seven legions were sent to end the rebellion in a single battle. With a spearhead force of Raven Guard, Salamanders and the Iron Hands' Morlock elite initiating the first strike, the others were sent in to back their strike. Unbeknownst to all however, almost the entirety of the legions backing them up had turned traitor, ready to join Horus in one moment. In a grim reflection of the Ullanor campaign, Horus proved his strategic genius, routing the Imperial forces, decimating two entire legions and taking the life of a primarch.
Even in M41 the effects of this are still being felt, as a number of legions regard that event with bitter distaste. It influenced the future of each force involved, sparked up rivalries and even led to fractures forming among the loyalist forces. However, recent lore has continually retconned the Iron Hands into blaming Ferrus for failing and dying. They are written now to almost openly deride him for charging forwards into battle, all the while blaming the other legions for his death. A contradictory stance to be sure, but the question is if he was truly to blame for his fate. The bigger question is that, if the other legions had followed him, what might have changed?
The key point to cite here is when the supposed second wave of loyalists arrived, the Raven Guard and Salamanders fell back while the Iron Hands pressed forwards. This has been blamed by some as Ferrus' hatred and bloodlust leading to his death, not acting with the others and effectively rushing right onto Fulgrim's sword. The thing is though, what would have been changed if he had retreated as the others urged? The two other legions were massacred with the massed firepower of their "allies", in an act of bloodshed which took the lives of tens of thousands of astartes and crippled almost the entire Imperial force. It was a group of already bloodied legionaries being attacked by masses of fresh, new forces, all with far more heavy armour to support them.
Given that the Iron Hands were far fewer in number in this conflict, adding them into the mix would have changed nothing. While it is true that a spearhead of Morlocks might have been able to break the traitor lines, they were already surrounded with the legions scattered, the Salamanders broken up and sent off to certain locations while the Iron Hands support elements and Raven Guard went to others. They were too spread out to capitalise upon any break-through attempt, or back the spear-tip of any assault, and even if they did what might have been accomplished? They would have still been surrounded, still have been facing the mercy of the traitor's guns and accomplished little beyond a few dead astartes in exchange. What's more is that, even arguing this, even assuming that the legions were in the right position to back an assault, the Morlocks and Ferrus were on the opposite side of the battlefield from the direction the loyalists were moving in. Unless they abruptly stopped, completely reforming their lines to have their best fighters charging towards their supposed allies, the Iron Hands would have arrived last.
Even the accomplishments people have suggested could have been pulled off with Ferrus' support are somewhat dubious. The foremost among these is that more of the traitor primarchs might have been cut down. An understandable claim to make, but for the most part one which would have been difficult to say the least. Think about who was involved for a moment and where their positions would have been. Perturabo, so far as most know, was sticking behind his artillery and Alpharius would have been impossible to find even on a good day. What's more is that Lorgar and Night Haunter would have been out of sight behind their lines were it not for the Word Bearers' primarch feeling a moment of compassion for his warriors. Well, that and Night Haunter having a sudden moment of strange behaviour. The problem is that, even if they were still to perform those same actions with Ferrus falling back, he and Vulkan would still be out of range to capitalise upon this.
To be completely blunt, Ferrus falling back would have earned them nothing beyond a few more dead loyalists to the guns of traitors. However, with that considered, what might have happened if the Raven Guard and Salamanders had opted to maintain their assault? Well, in all honesty, the Heresy might have ended there and then.
Think about it for a minute. Horus' plan had hinged upon them falling back as planned to continue the fight, and it had been Ferrus who had wanted to keep up the momentum of their assault. He was relying upon the bloodied loyalist forces pulling back and playing the long game, sticking to their initial gambit of one first wave breaking his defensive lines before a second followed up to finish them. At the time the Iron Warriors, Word Bearers, Night Lords and Alpha Legion arrived, the initial four legions had lost ground, a considerable number of legionaries, their defensive lines were in ruins and the Legio Mortis' assaults had been repeatedly deflected. While still a powerful force, they were in full retreat and much of their heavy armour was gone. If they had maintained their attack, rather than being outnumbered by fresh new forces, the loyalists would have still been fighting severely depleted traitor forces who had already taken a severe pounding. A move which, ultimately, would have likely reduced their overall casualties even without accounting for positioning, differences in firepower and ammunition loads compared with the arriving legions.
Think also of what this situation would have also meant when compared with the points outlined above. We've already established that they would have all been focusing their forces upon a single location, but think of the primarchs and the ease they could be reached. Those present in the initial landings were best known for leading from the front, with Angron, Mortarion and Fulgrim all seeking glory, blood or the pleasure of kills by their own hands. They would have been far easier to coax out of hiding or cut down while falling back, with Fulgrim in particular already exposing himself by moving out only with his elite guard. From the fight itself, we already know that Ferrus was on the verge of killing the primarch of the Emperor's Children, forcing the daemon corrupting him to directly take control and finish the fight. If even Corax or Vulkan alone were present to take him out faster or finish the job, or perhaps even just a few more astartes there to help cut down his bodyguard, the duel could have easily turned the other way.
Fulgrim's death alone would have dramatically shifted the direction of the Heresy. Without him the Iron Warriors would have never been diverted en mass to the Eye of Terror and later Tallarn. Atop of this, without his ascension to daemonhood and knowledge, Horus would not have attained the power he later gained from bargaining with the Chaos Gods. Thus, when facing off against the Emperor, he would have been at a severe disadvantage. This is also just one example. Atop of this, without Angron leading them the World Eaters would have become increasingly uncontrolled and directionless as an army, making them an even more unreliable in war. Hell, it's confirmed that Angron himself was key to forming the Ruinstorm. With him gone, the Ultramarines would not have been trapped. These are two key figures, both of who could have been drawn out and individually teamed up against, killed, and then had the loyalists move on.
However, consider the bigger prize here: Horus. While he might have been many things from a charismatic leader to a tragic figure, there is no denying that Horus was the linchpin holding everything together. He had been the one to speak to most of the primarchs, making deals, convincing them to side with him and bargaining however he could. While Lorgar was the power behind the throne, Horus was the one to sit on it, and without him the entire Heresy would have faltered. Think about it for a second, do you honestly not see Angron, Fulgrim, Mortarion, Curze, or even Purturabo and Alpharius not butting heads over who next led or what the pecking order was? Do you actually see them all willingly following Lorgar with the same willingness as they had shown Horus? Without him it's safe to say that the entire plan would have broken down, perhaps even caused Chaos to fragment as an allied force.
While each of the primarchs would have fought like the demi-gods of old, they would have been far easier to reach, far easier to find and ultimately far easier to directly engage. They would have actually been easier by comparison to target and annihilate despite a still admittedly high risk. Atop of all this however, charging forwards would have also unintentionally denied the traitors their biggest assets. If the fighting was close enough to the four main legions, they would have been unwilling to use their more powerful long range weapons thanks to the risk of friendly fire. Unlike what Chaos would become, they did show they weren't quite willing to go to the same lengths of disregarding the lives of their own forces, and any primarch who did suffer casualties would likely also harbor a grudge. This is also just accounting for traditional weapons however, and we do know far more powerful ones were present. Fulgrim, after all, noted a mushroom cloud billowing up from Vulkan's last known position.
Finally, win or lose, gathered at the fortress they would have been in a far stronger position to try and withdraw when the betrayal was commenced. A location where they were all united, distanced from enemy forces and could do more to try and keep the skies clear. Casualties would have still been staggeringly high even in this situation with desperate extractions, but they would have stood a better chance and pulling it off.
Personally, I stand by the argument that the Drop Site Massacre was a no win scenario. The loyalists were dead the moment they entered the system and, short of spontaneously deciding to leave upon arrival, there's no ending here which would not have been a staggering blow against the Imperium. Three loyal legions would have still taken horrific casualties, half the legions would have turned traitor and the attack would not have ended the rebellion fully as the Emperor hoped. However, it's hard not to see how the long derided and criticised move by Ferrus might have better benefited the Imperium than if things had played out as they did. It's just one of those moments in the Heresy where a single decision would have led to consequences capable of utterly reshaping the galaxy. One so small that, in all other cases, it goes unremarked and ignored.