A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
2 Chronicles 12:1
ch2 12:1REHOBOAM, FORSAKING GOD, IS PUNISHED BY SHISHAK. (Ch2 12:1-12)
when Rehoboam had established the kingdom, and had strengthened himself--(See on Ch2 11:17). During the first three years of his reign his royal influence was exerted in the encouragement of the true religion. Security and ease led to religious decline, which, in the fourth year, ended in open apostasy. The example of the court was speedily followed by his subjects, for "all Israel was with him," that is, the people in his own kingdom. The very next year, the fifth of his reign, punishment was inflicted by the invasion of Shishak.
2 Chronicles 12:2
ch2 12:2Shishak king of Egypt came up against Jerusalem--He was the first king of the twenty-second or Bubastic Dynasty. What was the immediate cause of this invasion? Whether it was in resentment for some provocation from the king of Judah, or in pursuance of ambitious views of conquest, is not said. But the invading army was a vast horde, for Shishak brought along with his native Egyptians an immense number of foreign auxiliaries.
2 Chronicles 12:3
ch2 12:3the Lubims--the Libyans of northeastern Africa.
the Sukkiims--Some think these were the Kenite Arabs, dwellers in tents, but others maintain more justly that these were Arab troglodytes, who inhabited the caverns of a mountain range on the western coast of the Red Sea.
and the Ethiopians--from the regions south of Egypt. By the overwhelming force of numbers, they took the fortresses of Judah which had been recently put in a state of defense, and marched to lay siege to the capital. While Shishak and his army was before Jerusalem, the prophet Shemaiah addressed Rehoboam and the princes, tracing this calamity to the national apostasy and threatening them with utter destruction in consequence of having forsaken God (Ch2 12:6).
2 Chronicles 12:6
ch2 12:6the princes of Israel--(compare Ch2 12:5, "the princes of Judah").
2 Chronicles 12:7
ch2 12:7when the Lord saw that they humbled themselves--Their repentance and contrition was followed by the best effects; for Shemaiah was commissioned to announce that the phial of divine judgment would not be fully poured out on them--that the entire overthrow of the kingdom of Judah would not take place at that time, nor through the agency of Shishak; and yet, although it should enjoy a respite from total subversion, [Judah] should become a tributary province of Egypt in order that the people might learn how much lighter and better is the service of God than that of idolatrous foreign despots.
2 Chronicles 12:9
ch2 12:9So Shishak . . . came up against Jerusalem--After the parenthetical clause (Ch2 12:5-8) describing the feelings and state of the beleaguered court, the historian resumes his narrative of the attack upon Jerusalem, and the consequent pillage both of the temple and the palace.
he took all--that is, everything valuable he found. The cost of the targets and shields has been estimated at about Â£239,000 [NAPIER, Ancient Workers in Metal].
the shields of gold--made by Solomon, were kept in the house of the forest of Lebanon (Ch2 9:16). They seem to have been borne, like maces, by the guards of the palace, when they attended the king to the temple or on other public processions. Those splendid insignia having been plundered by the Egyptian conqueror, others were made of inferior metal and kept in the guard room of the palace, to be ready for use; as, notwithstanding the tarnished glory of the court, the old state etiquette was kept up on public and solemn occasions. An account of this conquest of Judah, with the name of "king of Judah" in the cartouche of the principal captive, according to the interpreters, is carved and written in hieroglyphics on the walls of the great palace of Karnak, where it may be seen at the present day. This sculpture is about twenty-seven hundred years old, and is of peculiar interest as a striking testimony from Egypt to the truth of Scripture history.
2 Chronicles 12:12
ch2 12:12when he humbled himself, the wrath of the Lord turned from him--The promise (Ch2 12:7) was verified. Divine providence preserved the kingdom in existence, a reformation was made in the court, while true religion and piety were diffused throughout the land.
2 Chronicles 12:13
ch2 12:13HIS REIGN AND DEATH. (Ch2 12:13-16)
Rehoboam strengthened . . . and reigned--The Egyptian invasion had been a mere predatory expedition, not extending beyond the limits of Judah, and probably, ere long, repelled by the invaded. Rehoboam's government acquired new life and vigor by the general revival of true religion, and his reign continued many years after the departure of Shishak. But
he prepared not his heart to seek the Lord--that is, he did not adhere firmly to the good course of reformation he had begun, "and he did evil," for through the unhappy influence of his mother, a heathen foreigner, he had no doubt received in his youth a strong bias towards idolatry (see on Kg1 14:21).