That military situation is explained here:
Confusion and finger-pointing continue between local and federal law enforcement agencies after Wednesday's siege of the Capitol by a pro-Trump mob.
www.nbcnews.com
I would quote directly but it's a short read and explains the military side of things, or as it appeared to them following the request of the Mayor, about a month before.
So with that (the article above), from what I read, the National Guard would have easily been ready to handle the situation but they were scheduled ahead of time by the Mayor's request to simply act as support for traffic duties, and were to maintain a low profile, and were to only serve as de-escalation for any violent situations that popped up. It seemed like the Mayor expected the police to be able to adequately handle the situation on their own, for the most part. But we know the protests before Jan 6 were pretty violent and rowdy, so that's where the flag is raised. Because the police were not ready for the situation at all, and thus, what you saw on Jan. 6 occurs. I think the primary blame is on the police side of things from all that I have read, hell, things were shady leading up to Jan 6., seemed like the average local was more prepared for Jan 6 than the police were.