Is for evil men (and women) to fully embrace evil, that's what. You have to have that to have evil do anything. Evil men and women must embrace evil, sell their selves to the powers of Hell, and become instruments of the demonic and the damned. That's the first thin you need for evil to triumph.
The second thing necessary is for good men not to do nothing. In fact, people seldom do nothing. They almost always do something. And the second thing needed for evil to triumph is for otherwise good men (and women) to jump on the evil bandwagon. If the Nazi leadership was the full embrace of evil, those guys gunning down naked Jews in trenches were the ones who made it possible. And in most pictures, they aren't weeping either. They're grinning big grins as they're ready to massacre the innocent. That's the next thing needed for evil to triumph. Those who were not initially part of the evil jump into evil with both feet.
So is the next thing the do nothing good men? No. Again, men seldom do nothing. The next thing needed is for the good men who see what is happening to essentially make lame excuses or actually side with the evil, though they technically know it's wrong and don't support it. That's happening a lot today. I see people admit they see the clouds on the horizon, but at the first chance, they will excuse this or that: Well, in that case the police were wrong, or maybe banning a book with really bad content is good, or maybe it's right to point out that an entire ethnicity might always be guilty of something when there is a good argument. They know it's BS, and they often say so. But boy do they look for the chance to excuse surrendering the latest yard.
Now we're getting close. The next group isn't making lame excuses, but they're not doing much to stop it. They're not doing nothing. They see the evil, the problems, the racism, the tyranny, the shadows - but they're just not doing much about it, but not by doing nothing. They're fighting it, at least on paper. But with all the passion of a limp noodle. Perhaps they don't want to be mocked or laughed at, or worse lose their situation, but they draw a line between what they will do - fuss a lot on the Internet perhaps - and what they won't do (jeopardize their livelihoods).
Finally, when all that is done, I'm sure there might be the odd individual who does nothing. If that's the case, however, it's often that they're simply ignorant, or one of the masses of uninformed just trying to eke out a living and with no time to do more than look at the occasional headline. They're the ones in the rural countryside who probably never heard of National Socialists in the German government in 1933. By 1945, however they had heard. By then, of course, it was too late.
So there you have it. As we see the officially sanctioned racism and discrimination, as we see the hate and endorsed vengeance, as we see the anti-American rage and destruction, the killings and attacks, the tyranny and oppression, the open calls to destroy everything to do with the Christian West, the excuse making to support bearing down on Christ's Church, we wonder how it's happening and happening so quickly. That's not because good men are doing nothing. It's because evil men (and women) are doing what they do, and that's pushing evil. The rest are falling into their own category of being active enablers of the evil. Just as men and women have done for eons. And just, as we're seeing, they will no doubt continue to do for eons to come.