The writer repeats what he had said in 1-Kings 14:30, in order to remind the reader that Abijam inherited this war from his father. Abijam's war is described in marginal reference That the author of Kings gives none of its details is agreeable to his common practice in mere military matters. Thus he gives no details of Shishak's expedition, and omits Zerah's expedition altogether.

There was war between Rehoboam and Jeroboam - This was mentioned in the preceding chapter, 1-Kings 14:30, and it can mean no more than this: there was a continual spirit of hostility kept up between the two kingdoms, and no doubt frequent skirmishing between bordering parties; but it never broke out into open war, for this was particularly forbidden. See 1-Kings 12:24. Hostility did exist, and no doubt frequent skirmishes; but open war and pitched battles there were none.
But why is this circumstance repeated, and the history of Abijam interrupted by the repetition? There is some reason to believe that Rehoboam is not the true reading, and that it should be Abijam: "Now there was war between Abijam and Jeroboam all the days of his life." And this is the reading of fourteen of Kennicott's and De Rossi's MSS. The Syriac has Abia the son of Rehoboam; the Arabic has Abijam. In the Septuagint the whole verse is omitted in the London Polyglot, but it is extant in those of Complutum and Antwerp. Some copies of the Targum have Abijam also, and the Editio Princeps of the Vulgate has Abia. This is doubtless the true reading, as we know there was a very memorable war between Abia and Jeroboam; see it particularly described 2-Chronicles 13:3 (note), etc.

And there was war between Rehoboam and Jeroboam all the days of his life. Not the days of Rehoboam, though that was true, and is observed, 1-Kings 14:30, but all the days of Abijam, before he came to the throne, and in which, when a young man, he was concerned, and which still continued between him and Jeroboam; though some think he is called by his father's name, as Rehoboam is called David, 1-Kings 12:16.

"And there was war between Rehoboam and Jeroboam all his life;" i.e., the state of hostility which had already existed between Rehoboam and Jeroboam continued "all the days of his life," or so long as Abijam lived and reigned. If we take חיּיו כּל־ימי in this manner (not כּל־ימיהם, 1-Kings 15:16), the statement loses the strangeness which it has at first sight, and harmonizes very well with that in 1-Kings 15:7, that there was also war between Abijam and Jeroboam. Under Abijam it assumed the form of a serious war, in which Jeroboam sustained a great defeat (see 2 Chron 13:3-20). - The other notices concerning Abijam in 1-Kings 15:7, 1-Kings 15:8 are the same as in the case of Rehoboam in 1-Kings 14:29, 1-Kings 14:31.

War between, &c. - Upon Jeroboam's invading him with a great army: acting then in his own defence, he totally routed Jeroboam, so that he was quiet the rest of his reign.