Jamieson, Fausset and Brown on 1 Chronicles 12:16

16. the children of Benjamin and Judah--It is probable that the Benjamites invited the Judahites to accompany them, in order to prevent David being suspicious of them. Their anticipations, as the result showed, were well founded. He did suspect them, but the doubts of David as to their object in repairing to him, were promptly dispelled by Amasai or Amasa, who, by the secret impulse of the Spirit, assured him of their strong attachment and their zealous service from a unanimous conviction that his cause was owned and blessed of God (1Sa 18:12-14).