American history[1] shows this probably isn't a requirement - beyond a foreign military attacking, which is clearly out of scope of policing, the other attacks were acts carried out by isolated people. "Storming the gates" hasn't happened before now.
I suspect the main reason that this hasn't happened before is that very large protests/gatherings are often met with a large show of police force to ensure the protestors know this isn't an option. Why that didn't happen today will be interesting to investigate. We all probably have a theory, but what comes out of the inevitable hearings on this will be interesting to see.
There are hundreds of thousands of people behind the camera, going all the way to the Lincoln Memorial. You can see some security milling around, but no large show of police force.
The Million Man March's attendance didn't include people with an established history of bringing weapons and wearing body armor at ostensibly peaceful demonstrations.
That couldn't be further from the truth. A large group of Black Panthers armed with assault weapons stormed the CA capitol in 1967 as part of a protest.
I suspect the main reason that this hasn't happened before is that very large protests/gatherings are often met with a large show of police force to ensure the protestors know this isn't an option. Why that didn't happen today will be interesting to investigate. We all probably have a theory, but what comes out of the inevitable hearings on this will be interesting to see.
[1]: https://www.cnn.com/2021/01/07/us/us-capitol-violence-histor...