The fact of the matter is that its a necessary evil as a war deterrent and protection.
I know what some of you will say, that if the need arose people could just partake in guerrilla warfare.
But history has shown that in asymmetrical warfare the guerrilla side always take the most casualties by a very high magnitude.
Also we cant ignore the fact that in the absence of a standing army there'd be warring factions withing the land.
So for pragmatism's sake there should be a standing army.
As far as law and order, that would be up to the communes.
But there would probably be a lot less crime in anarchist society in which the worker's owned the means of production.