Verses 13 and 14 clarify this causitory position of verse 11. The destruction will come upon Israel and Syria because the people have failed to turn to the lord. For that reason they will be "cut off" (verse 14).
Precisely what will be "cut off" is explained with two metaphors "head and tail" and "branch and rush." Each of these is intended to show the completeness of the "cutting off." Head and tail are the extremities of the animal, and may stand for the entire animal. Branch and rush refer to both the developed branch of a tree and the seedlings from which the tree will grow. The destruction will be of the whole. Of course there is a small remnant that is also predicted, and perhaps this is also imaged in that the metaphors do not explicitly mark the whole.