A modern reader would find nothing surprising in this treachery, but in the ancient world it is a startling betrayal. Had a promise held only ephemeral value, Alma would not have believed it in the first place and would have sent the army in the wrong direction.
In essence, Mormon describes a promise made and a promise broken. However, it is possible that Mormon misunderstood part of the negotiations between the Lamanite leader army and Alma “and his brethren” (which would have included Helam). Or perhaps Joseph Smith mistook the complexity of the situation. What was promised was liberty—which might mean something different to modern readers than it did to the ancient Mesoamericans. The Almaites’ lives were spared. Perhaps that was part of the liberty.
The Lamanites were setting up a tribute-paying arrangement, as they had done with the Limhites earlier, enforced by surrounding guards. The repetition suggests that such a practice was a standard part of subjugation. Although the translation appears to describe treachery, the actual details suggest that they were merely following the standard procedure in such a situation. It was a procedure that Alma and his brethren would have understood.