First, my understanding of Posse Comitatus Act is that the US military cannot be used against US citizens. These are not US citizens. By your definition we would NEVER be able to use the US military to protect our domestic border?
Secondly, my response was regarding the poster who said we our military could not be ready to protect our border within 3 weeks. Again, that seems absurd considering what if we were attacked by a foreign government - we would not be able to protect our border for 3 weeks? That doesn't make sense to me.
It could be done with an act of Congress, but they're all out campaigning and likely wouldn't come back for this, because the military is limited in how helpful it can be.
It's just like if you have a new hire at work. 80% of the work, the military won't be authorized, trained, familiar with. When we've done this in the past, they chiefly provide logistical support, which is great, but there's only so much logistical work that needs to be done.
Believe me, there's nothing I'd like better than to help our brave men and women of CBP and Border Patrol, this could be a terrible situation, but we needed staffing two years ago. It's too late.