An overreaction, to say the least.
For weeks now a group of Central and South American migrants have been walking towards the United States in growing numbers with the hopes that they will be allowed to exercise their legal right to seek asylum in the US. Unfortunately, it's looking less and less like there will be safety in numbers and more like something scary is going to happen.
President Trump has, to say the least, been exploiting the caravan by making it into a sensationalized wedge issue that paints the migrants themselves as barbarians coming to storm the gates of beautiful, civilized America (a good, non-partisan radio report can be found here). He dialled this rhetoric up a notch today gave a characteristically will-he-or-won't-he threat to the migrants, saying that if they throw rocks at American border guards (which presumably they would do only if denied entry), then Trump says the American forces will consider it as if they were shooting guns and will not hesitate to shoot back.
Anybody throwing stones, rocks, like they did to Mexico, the Mexican military, the Mexican police, where they badly hurt police and soldiers, of Mexico, we will consider that a firearm, because there's not much difference...they wanna throw rocks at our military, our military fights back
He went on to clarify how he phrased it to the military: "I told them: 'consider it a rifle.'" Let's hope that the military of a country founded by immigrants shows some restraint.