> Who will enforce any kinds of laws?
Maybe you and me, and our neighbors, as a community. Why would we concentrate police powers in the hand of a specialized militia answering only to the psychopaths in government? Cops are never here when we need them to protect actual people, but they always show up and mess everything up when things were getting better.
Laws are written in a social context. You can break laws without harming anyone, in which case nobody in your neighborhood may mind (and maybe some people would like to repel that law at the next general assembly). If some people think you are harming others, then there are many ways to try and find an arrangement to the situation and repair any wrongdoing (see also: reparative/transformative justice).
Of course, some extreme cases require use of violence to control a person who's physically endangering others. Why would we make a job out of it, though? Concentrating those powers and responsibilities into a finite set of hands sounds like a fragile system that can be abused.
See also: past HN threads about the positive outcomes of not involving police for mental episodes, accounts/studies of indigenous systems of justice (such as in Chiapas), and academic research on the abolition of prisons (Angela Davis, Gwenola Ricordeau).
> how can anyone that has heard of any amount of history take seriously the idea that a country like the US can just abolish its military without it being in catastrophic danger?
The US is the catastrophic danger to many countries (Venezuela, Cuba, Iran, etc). By dismantling the US military industrial complex, we'd be doing a lot to promote peace and stability across the entire globe. The USA is still by far the largest military in the world and has helped countless coup d'État and assassinations. If only those, the war files released by Wikileaks and the recent debacles in Iraq and Afghanistan should be good examples of the military producing the exact opposite of its stated goals.
> If the abstraction of money didn't exist, what would happen exactly?
It depends on a wide variety of social/cultural factors. There's not a one-size-fits-all way of life, and each community may adopt different approaches to property and commercial exchanges. But in my opinion and experience taking away the abstraction allows us to focus on actual concrete problems.
For example, think about housing: by reasoning in terms of money, you reach the conclusion that we need more investment in new housing. By thinking without money, you can notice empty dwellings outnumber homeless people > 10-to-1, so the question becomes: if we have enough housing for everyone, why would we let people be homeless?