Falerii was conquered by the Romans shortly before the Gallic invasion which concludes Livy's story. Its conquest though is worth pausing over for a minute because it gives us an idea of what Livy (and his audience) thought were the laws of war. Falerii was surrounded by Camillus. Having been surrounded, we are then told by Livy that a school master to the senior men of the town took his charges for strolls of greater or lesser extent.
One day he saw his chance for a longer stroll than usual and took his young charges right through the enemy camp to Camillus's tent. (V 27)
The obvious consequence was that Rome for a moment held important hostages and could have potentially forced the surrender of the city, but Camillus refused to do so. Rather he addressed the schoolmaster,
Neither my people nor I, who command their army, happen to share your tastes. You are a scoundrel and your offer is unworthy of you. As political entities there is no bond of unity between Rome and Falerii, but we are bound nonetheless and always will be by the bonds of a common humanity. War has its laws as peace has, and we have learned to wage war with decency no less than with courage. We have drawn the sword not against children who even in the sack of cities are spared, but against men, armed like ourselves.... These men, your countrymen, you have done your best to humiliate by this vile and unprecedented act: but I shall bring them low... by the Roman arts of courage, persistance and arms. (V 27)
Camillus thus sent the schoolmaster and his charges back to the city- the boys whipping the schoolmaster as they went. The citizens of Falerii were so impressed that according to Livy they surrendered the city immediatly to the Roman commander.
As ever who knows how true this story is. But the important point is not the story- the capture of Falerii was not a world changing event- but the point that Livy seeks to make through the story. Camillus here represents the ideal Roman response to the position he was placed in. The ideal Roman response was to reject the offer- for two key reasons. Firstly and this is important to understand, this story illustrates the boundaries of Roman doctrines of war. It demonstrates that Livy and his audience thought that children should be excluded from war as a matter of course. Secondly it illustrates the degree to which such strategems were thought to be opposite to the kind of courage that Livy and Camillus see as a political virtue. The laws of war set out expectations for each side and thus allow both to anticipate the other's moves- the arts of war are the arts of courage- and can be united in the adjective Roman because of this. War takes place in the open.
Lastly it is worth noting that Livy attaches a reward to following these laws. Good behaviour produces good results- this is very notable in Livy's discussions of religion as well. You do the right thing and you are rewarded for it. In this sense Livy's conception of morality- something to follow which is right and will give you success- differs from some modern understandings of morality where you do something because it is right and irrespective of whether it delivers success. In that sense we are closer to the medievals than the classics.