Firstly, it is known that Sargon II’s artists made numerous reliefs in his palace at Room V at Dur-Sharrukin. Several of these reliefs are extremely helpful in the precise dating of the rise and falls of certain cities in Philistia. The first slabs show the Raphia and Samaria campaigns of 720 BC, on the lower and upper registers respectively. On the lower register, after Raphia, defended by Nubians, comes Gibbethon, also defended by Nubians. Whether this is Philistine Gibbethon or a place near Raphia is uncertain. After Gibbethon, a large city (some 70-150 acres), with an acropolis, is seen in Slab 6. No city north of Gibbethon is this large, and the only candidates are Gaza, Gath, and Ashdod. Gath’s walls do not, as in the relief, extend to the valley, the only candidates are, therefore, Ashdod and Gaza. By the appearance of the relief, it appears to be Ashdod, but, by the fact Ashdod is not mentioned in the Sargonite annals of 720 BC, around which campaign much of the room seems to revolve around (although they may date to the 711/12 campaign, but Ekron is not mentioned in the accounts of this campaign), but Gaza is, it appears to be the latter. On Slab 11, Ekron, pictured as a small city, and definitely the small, fortified acropolis of Stratum II, is pictured below Sargon’s 720 BC Hamath campaign.
In short, most of the reliefs in Room V relate to the 720 BC campaign of Sargon II.
Secondly, it is known that, in 712/11 BC (14th/15th year of Hezekiah), Sargon’s Tartan captured Ashdod. According to Sargon himself, after Yamani fled, he captured on this campaign Ashdod, Gath, and Ashdod-Yam (Tel Mor, on the N. bank of the Nahal Lachish, some 1400 meters from its mouth). These cities are mentioned entirely in the context of the Ashdodite campaign, and show that Ashdod ruled Gath (stratum F8) before 711/12 BC. Neither Biblical nor Assyrian source mentions any other kingdom in the area of Judah this campaign was fought against.
Thirdly, it is known that, in 701 BC, Sennacherib fought with the Nubians at Eltekeh (Tell esh-Shalaf?= 31°53’35″N, 34°46’6″E) and took over the kingdom of Ekron, re-instating Padi (taken hostage by Hezekiah) as king, and captured forty-five of the fortified cities of Judah, most notably, Lachish Stratum III.
The “Azekah Inscription” is a damaged Assyrian tablet, mentioning a curious form of the god Ashur as “Ashan”, and the conquest of two cities, firstly, Azekah, and “a royal city of the Philistines, which Hezekiah had captured and strengthened for himself”.
There are several reasons why the Azekah Inscription far more likely came from the reign of Sennacherib than that of Sargon II. Firstly, the 720 BC campaign of Sargon nowhere mentions any city of Judah, although it was very varied in its purpose (Hamath, Samaria, Raphia, Gaza, Ekron), it seems that Hezekiah, if anything, supported this campaign (2 Kings 18:8?). Also, since Ekron, portrayed in Sargon’s reliefs as a small city, could not have been an inspiration for the Azekah text, and there is not the least bit of archaeological or inscriptional evidence for Gath being a re-fortified Judahite city in 720 BC, it is extremely implausible the 720 campaign inspired the Azekah text. The 712/11 campaign campaign is definitely not the campaign that inspired the Azekah text, for obvious reasons (Gath an Ashdodite city, Tartan, not Sargon campaigned, no mention of Ekron, in any case). The result of all this negative evidence is that the Azekah Inscription must reflect the 701 BC campaign of Sennacherib.