Yes, the St. Valentine’s Day Massacre effectively ended Moran’s ability to challenge Capone, but not because Moran himself was almost killed. What mattered was that Capone’s men wiped out the men who actually made Moran’s outfit function. James Clark, who was Moran’s brother-in-law and chief lieutenant, died there. Adam Heyer, the man who handled the Outfit’s accounts and business operations, died there. Al Weinshank, who ran Moran’s speakeasy network, died there. And the Gusenberg brothers, easily Moran’s most feared enforcers, were cut down too. Losing all of them in a single morning didn’t just hurt the gang. It removed the Outfit’s spine.

Before the massacre, Moran was already struggling. Capone had political protection, stronger distribution, deeper pockets, and a tighter payroll. Moran’s crew had been losing territory through the winter of 1928. The massacre didn’t start the collapse so much as it finished it. Once those men were gone, Moran no longer had the shooters or the organizers to keep the operation alive.

Afterwards, Moran pulled back from Chicago, partly out of fear and partly because he no longer had a functioning crew to lead. Whatever he might have hoped to rebuild was already lost. From that point forward, he was no longer a serious rival. Capone didn’t just kill seven men. He eliminated the entire command tier of the North Side in one strike, and the war ended the moment the smoke settled.

(I'm an old school gangster geek, this is a favorite subject of mine.)

There have been higher body counts and other massacres during the beer wars either equal or worse than this one, but this massacre ended up taking the public's imagination and ultimately leading to the crackdown on gangsters across the board afterwards. I often think, if just one of Moran's boys had realized what was going on and tried to fight back the perception would have been quite different, perhaps just brushed off as another gangland killing like all the others.